148 LECTURES ON 



tracts of country covered with barren, drifting sands, on whose arid 

 bosom only a few stunted pines or shrivelled grasses find nourish- 

 ment. Again there occur in the highlands of Scotland, Bavaria. 

 Prussia, and other temperate countries, enormous stretches of moor- 

 land, bearing a nearly useless growth of heath or moss. In Southern 

 Russia occurs a vast tract, two hundred millions of acres in extent, 

 of the tschornosem, or black earth, which is remarkable for its extra- 

 ordinary and persistent fertility. The prairies of our own west, the 

 bottom lands of the Scioto and other rivers of Ohio, are other ex- 

 amples of peculiar soils; while on every farm, almost, may be found 

 numerous gradations from clay to sand, from vegetable mould to 

 o- r avel gradations in color, consistence, composition, and produc- 

 tiveness. 



Some consideration of the origin of soils is adapted to assist in 

 understanding the reasons of their fertility. Geological studies give 

 us reasons to believe that what is now soil was once, in chief part, 

 solid rock. We find in nearly all soils fragments of rock, recogniza- 

 ble as such by the eye, and by help of the microscope it is often easy 

 to perceive that those portions of the soil which are impalpable to 

 the feel are only minuter grains of the same rock. 



We have space for only the merest general outline of what was 

 probably the original condition of the earth, and of the successive 

 'changes that have wrought it to its present state. During the lapse 

 of the uncounted ages that have been forming our globe, rocks have 

 been ground to soil, and soil has been recemented into rock, and to- 

 day the same transformations are slowly and silently proceeding. 

 When the earth first cooled down from the primal heat, it had no 

 soil, in the proper sense of that word, but was a mass of crystalline 

 granitic rocks and volcanic scoria?, incapable of supporting vegeta- 

 tion. When the vapors condensed upon its surface, began that strife 

 between fire and water which, under the mild forms we call weather, 

 has never since ceased. Rains then began to fall upon the mountain 

 wrinkles produced by the contraction of the cooling crust. Streams 

 flowed downward into the valleys, cracking the still hot rock, whirling- 

 fragments along in their courses until they settled as gravel, sand, or 

 finer powder to the bottom of some quiet sea, or were dissolved in 

 boiling wells. In later epochs vegetation began to flourish; then, 

 after slow centuries had passed, animal life was set in process; each 

 department of organized existence, in its own way, adding to the list 

 of changes. From the first the atmospheric oxygen was omnipresent, 

 and carbonic acid too, began to act upon the rocks; and as the result 

 of the solvent, decomposing, breaking-up, and commingling course of 

 operations, thus carried on through long periods of continual action, 

 we have the soil in its present characters and aspects. 



The mechanical force of running water has been among the most 

 effective agencies in the pulverization of rocks. During what is 

 termed the diluvial or drift period, a current of water passed from 

 north to south over the northern portion of this continent, wearing 

 down the rocks, and bearing with it an enormous mass of solid matters, 

 which now remain as then deposited, constituting gravelly hills, and 



