« AnteriorContinuar »
magnesia to make it a true lithological dolomite. They are all merely more or less magnesian limestones.
5. Carbonate of magnesia is not absent from any bed in whole series ; but in an extensive range, (such as from No. 84 to No. 115,) out of thirtytwo beds, twelve show less than two per cent., three show less than three per cent., and only one goes up to four and six tenths per cent. The remaining sixteen beds, alternating with other sixteen with great regularity, carry from thirty-six to fourteen per cent.; nine of them ranging between thirty-six and thirty, five between thirty and twenty-five, one sinking to seventeen, and one to fourteen per cent.
The alternation in these thirty-two beds may be represented to the eye thus :
It is especially remarkable that so few of the beds occupy an intermediate position, chemically considered, between, almost pure limestones and almost pure dolomites.
The cause of all this is at present wholly conjectural, and speculation upon it is rendered all the more difficult and doubtful, by reason of the total absence of fossilized organic forms. We know that the seas of that remote age furnished a home for incredible numbers of invertebrate living beings; for the remains of cephalopods, gasteropods, brachiopods, articulates, lamellibranchs, and bryozoa obtained from these rocks fill the museums of Europe and America ; but they seem to have lived along the shores. Their remains are extremely scarce in Pennsylvania, where the sea seems to have been deep. Probably they were dissolved, in the manner described by Mr. Alex. Agassiz in his recent report on the animal forms obtained by dredging the bottom of the Carribean sea.
It must not be taken for granted that the whole mass of three thousand five hundred and thirty-five feet of magnesian limestones (as measured by Mr. Sanders along the Susquehanna river) consists of regularly alternating layers of dolomite and limestone, merely because we have proved
by the above analytical investigation that such alternations exist in a part of the mass more than four hundred feet thick; but since the layers studied occupy a place towards the middle of the mass, and are fortui. tously exposed opposite Harrisburg, it is certainly not an improbable supposition that the entire formation from top to bottom is subject to this remarkable and mysterious law. Such a law, whatever be its restrictions, demands the earnest attention of chemists and geologists.
But such a law imports much, also, to the agriculturist. Farmers will take a practical interest in it when they come to see its practical bearing on their system of mineral manuring.
I must remind you here that the geologist proper has nothing to do with the physiology of plants and animals, in relation to agriculture. Agri. cultural chemists take charge of this subject.
But it is necessary for the geologist to keep in view what plants require in the shape of food; and that this they get partly from the air and partly from the soil.
1. It seems to have been proved, by a suflicient number of experiments, that if a soil does not contain a sufficient quantity of each and all of the following minerals: Phosphoric acid, sulphuric acid, potash, lime, mag. nesia, oxide of iron, it may be considered a barren soil. Not only each of these, but all of them, are needful. If any one of them be absent from a given soil, seeds can not germinate, plants will not grow in it.
2. But these elements must also be present in a form which plants like. They must be, so to speak, digestible. Many fruits and roots of the earth are food for man only when they have been cooked. Animals live upon the same elements as plants, but only after the plants have prepared the elements to suit the constitutional digestion of animals. Plants require phosphorus; but if it be in the soil only in the form of apatite or crystalline phosphate of lime, the plants cannot use it. Hence, artificial manures are made out of ground apatite treated with sulphuric acid, to render the phosphoric acid soluble in water, and then plants can imbibe it. The "phosphatic guanos,” which are so extensively used by farmers, are made from nodules of apatite, found in the marls of South Carolina, and these are relics of the marine animals which lived in the geological age immediately preceding the present. The phosphate of lime in animals, their bones, has a texture which makes it much more soluble than the mineral apatite, and hence bone dust manure is better than mineral manure.
Plants require magnesia; but they will not take it from serpentine rocks, although these are largely made up of magnesia in combination with silica and water. But on magnesian limestones plants thrive.
Plants require potash, and potash is abundant in some of the feldspars which make up granite, gneiss, and sand rocks. But a plant would starve to death if it had to suck out the amount of potash it wanted from feldspar grains and crystals. On the other hand, such minerals as carnallite and sylvite, (Stassfurt,) containing large quantities of chloride of potassium, make a good manure. A still better form of potash, in a condition perfectly acceptable to plants, enabling them to absorb it with great ease, is glauconite, i. e. the green grains in the Jersey marls. The best green marl contains ten or twelve per cent. of potash (equivalent to the potash of commerce.) But the best of all forms in which potash can be offered as food to plants is that of wood ashes, or straw, or the leaves and twigs of trees; for the plant may then be said, poetically, to do what the cow does when she first chews the cud and then swallows and digests it. The ashes from burned wheat straw consists nearly one half (forty-seven per cent.) of potash; the ashes from oak leaves are nearly one quarter (twentyfour per cent.) potash. Potash makes up one sixth of the weight of maize stalk ashes ; more than one eighth of grapevine ashes; one twentieth of flax; one fourtieth of willow, and one hundredth of pine wood ashes. That is one reason why the annual droppings of a natural oak forest make a soil in which underbrush flourishes; while the soil of a pine forest presents so bare and sandy an appearance. It is not indeed the only reason, but it is an important one.
It is a remarkable fact, that the two most abundant mineral elements in our rocks, alumina and silica, the bases of sand and clay, are not used by plants at all as a part of their food necessary for sustaining their life, but only in a mechanical way to glaze or stiffen their stalks, or to mix with their bodies without either hurting or helping them.
Soda also is almost absent from the bodies of plants. Animals therefore, to whom it is quite indispensable, are obliged to get it in other ways; and hence wild animals betake themselves to salt licks, and tame animals grow fat when driven to pasture for a time on salt marshes.
As for nitrogen, which plants cannot do without, it can hardly be found in any rocks, and therefore must get into the soil not from below, but from above, viz: out of the air, (which consists of seventy-seven parts by weight of nitrogen, twenty-one of oxygen, and the rest of carbonic acid, watery vapor, and a very little ammonia.) Ammonia is itself a compound of nitrogen and hydrogen.
It is from the carbonic acid of the air that plants get the carbon with which to build up their bodies.
The ammonia of the atmosphere supplies plants with what nitrogen they need. In doing this, it first unites with sulphuric and other acids in the soil. Animals supply a large quantity of ammonia to vegetation in their urine. Hence the value of barn-yard and privy manure. The wild animals, large and small, are thus all the while manuring the forests and the fields without asking permission of the farmer. Every squirrel and ground hog helps to care for the healthy vigorous growth of the tree he lives on or under. The air deposits part of its nitrogen, in the form of saltpeter, in every cave and quiet nook, and under eaves, and boards, and fallen trees, where the salt is protected from the rain. And, indeed, everywhere else; only that in unprotected places the rain dissolves the niter, and washes it into the soil. Nitrogen is abundant in the hair and horns of animals. Ammonia got its popular name of “hartshorn” because it was at first distilled from horns. Although the mice eat up all the horns which deer shed and drop in the forest (so that by June, nothing is left of them but their tips) the ammonia is not lost to the soil, for the mice pass it through their own bodies to the earth, to fertilize which it is surely destined.
With this whole subject of the animal and vegetable manures, the geologist has nothing to do. It is not his business to discuss the compounds of carbon and water, which make wood-fiber, starch, sugar, and gum; nor the compounds of carbon, hydrogen, and oxygen, which make the pulp of fruits and roots; nor the fatty and oily parts of plants; nor those mysterious and beautiful acids which give taste to the apple, the lennon, the strawberry, and a thousand other products of the marsh, the forest, and the orchard; nor those more complicated compounds of carbon with the three gases, oxygen, hydrogen, and nitrogen, out of which come the milk of animals, the white of eggs, the fiber of muscles, the pulp of the brain.
All that the geologist has to deal with for the farmer's sake may be limited to what he finds in the ashes of plants: the phosphates, the sulphates, the chlorides, the silicates, the carbonates-of five metals: Potassium, sodium, calcium, magnesium, and iron.
These are the mineral constituents of the rocks, and of the subsoil; and these are the mineral constituents of the upper soil also, but are in it mixed with decaying and decayed vegetation : The roots and leaves of plants; the dung, manure, and dead bodies of animals; and the nitrite salts and ammoniacal compounds which rain water has brought into or developed in it.
One point, however, is very important, and it is this: Plants are, in general, exceedingly small eaters. Temperance, even abstemiousness, characterizes their society. The weeds are rather gluttonous; corn and timothy have good round appetites. But the trees live on next to nothing. All the plants, indeed, offer an amazing contrast to the voracity of the animals (especially the smaller ones) which eat them up, either alive or dead. The largest tree weighs but a few tons after living a thousand years. Most of its food has been a watery solution of charcoal, flavored with a little salt. It is this flavoring salt, or rather compound of the salts mentioned above, which inspires the curiosity of the geologist, and demands the study of the farmer, the study of years, regardless of trouble or expense.
An acre of ground may be said to have upon it four thousand tons of soil, if the soil be two foot deep. Of this, only forty pounds weight is removed in the shape of seed, by harvesting a crop of grain of thirty-three bushels; and one hundred pounds more in the shape of straw. A bay crop of two tons carries off only two hundred and sixty pounds of mineral matter.
Suppose a farmer should want to get from five hundred tons of perfectly barren soil, spread over an acre of ground, a crop of thirty-three bushels of barley, and one ton of straw, how much soluble potash, soda, magnesia, lime, phosphoric acid, sulphuric acid, chlorine, and nitrogen (in the form of nitrates) must he put into it, or find in it? Hellriegel, after much experimenting, was able to answer the question thus :
55 pounds of potash.
8 pounds of chlorine.
240 pounds in all, and (minus nitrogen) only
186 pounds of mineral substances. This will suffice to produce the 140 pounds of seed and straw required.
You see then what successful farming can be done on a very shallow depth of soil, if the farmer be a knowing man.
This brings me to the second part of my subject, viz:
2. That other essential feature of soil, as a geological phenomenon, namely, its depth.
I have spent so much time in speaking of the quality of soil, that I have only time enough left this evening to draw your attention to the geological fact that the depth of any kind of subsoil depends upon the character of the formation over which it lies, and out of the edges of whose beds it has been manufactured by the processes of nature.
In the banks of the Schuylkill, below Philadelphia, one can see how the feldspar rocks have been mouldered by weathering into subsoil to a depth of several yards. Along the foot hill country in front of the Blue Ridge, in Virginia and North Carolina, the rocks have been converted into subsoii to a depth of sometimes one hundred feet. Many of the clay shale beds of Middle Pennsylvania are decomposed thus to a great depth beneath the surface. This is the process that has converted our rock-fossil-ore-beds into soft fossil ore for one bundred or even two hundred feet down; while the sandrocks alongside of them have turned into subsoil for only a few inches. All kinds of rocks are thus mouldered, more or less, by the long continued action of the carbonic acid of rain water. But their powers of resistance differ immensely. Some outcrops make soil so slowly that they lose it by the washings of the weather as fast as they get it, and so they remain standing above the surface with a rugged and forlorn aspect, much to the discomfiture also of the farmer who owns them, and does not know what to do with them. This is what makes our mountains. Their rocks are indestructible, except by ice. They stand high in air, while the rocks of the valley formations are mouldering and lowering their richly soil-covered surface nearer and nearer to the level of the sea. Our observations teach us, beyond a doubt, that the surface of this limestone and slate valley of Cumberland and Dauphin county was, in past geological ages, many thousand feet higher above sea level than it is to-day. And you all know what destruction is still going on under the surface—how many hundreds of caverns underlie the farms of this district. It is easy, therefore, for the mind's eye of the geologist to see the whole of nature's process of manufacturing soils in perpetual activity.
Still one more geological point must not be forgotten.
The soil which lies on a dead level remains nearly stationary where it was made; that is, on top of the rock out of which it was made.
But the soil of a hillside is all the time creeping down the slope—very slowly, indeed, hut very surely, and all the time. Every time a summer shower saturates it, part of its elements are dissolved and percolate away, and the insoluble parts shift their place a little, and, of course, down hill. Every winter's frost expands the mass and shoves it a little further down hill. Any one can see the evidences of the operation if he chooses to look for them. The coal miner knows perfectly well that he must look for the blossom of a coal bed a good way down hill below where the bed lies in the rocks themselves. The whole surface of the earth is sliding down hill from the hill tops towards the valleys. All soils slip sideways from off their mother rocks down over the mother rocks below them. The geologist who is studying soils for the farmer must take account of this, and not mistake the soil
of one rock for that of another. I have occupied too much of your attention this evening to go on longer.
I should be glad to give, also, a succinct account of how far the geological survey of the State has advanced; but it would be a long story; for a great deal has been done. I must, therefore, refer you for this, to the official Reports of Progress published in separate volumes by the Board of Commissioners of the Survey.
I cannot tell how practical my remarks may seem to you to be, for they are necessarily mixed with much which is theoretical or scientific. This marvelous plant life, this universal development of intimate relationships between the inorganic and organic worlds, between geology and agriculture, is an exceedingly interesting subject. Whole volumes have been written on it, and placed in the libraries; and still this many-sided phenomenon of plant life on the planet takes such hold of our imagination and appeals so forcibly to all our human sympathies, that it is a great delight to attempt to picture and explain it to an intelligent audience. I thank you for the attention you have given me tais evening. [Applause.]
The Governor has just suggested that I should answer interrogatories,