18 GARDEN MANAGEMENT. 



rough and partially decomposed surface of the rocks, and, finding here suffi- 

 cient food, germinate, and throw out roots, which penetrate into the crevices 

 of the rocks like wedges, widening and separating them, and hastening their 

 decomposition ; for in their roots thej' retain the water, which finally acts 

 upon them by its dissolving powers. Insects come to feed upon the mosses 

 and lichens, and, finally, both die, leaving the rocky matter, originally purely 

 mineral, a mixture of animal, vegetable, and mineral remains, or humus. A 

 thin layer of fertile soil is thus formed, on which plants of a higher order 

 spring up, all tending to produce the mighty results we have hinted at, 



41. Wind, water, — above all gravitation, are the more mechanical agents in 

 the disintegration of rocks. Water, aided by the tempest, having washed 

 away all the softer supports of a mass of rock, this, in obedience to this irre- 

 sistible law, soon falls, contributing its mass to fill up the valley below, the 

 rocks being reduced in their fall to smaller pieces ; and, finally, obedient to the 

 chemical laws, they crumble to dust, more or less rapidly, according to their 

 nature and the atmospheric influences. 



42. Water, which thus acts as a chemical agent in destroying rocks, is also 

 strong in its physical force. As falling rain, it washes down all loose particles 

 into some river-bed, which again carries down the finer particles, held in 

 suspension, till deposited in the delta, to form, at some futui-e day, a field, a 

 district, or a new country, as the case may be — rescued from the flood. Again, 

 in the earlier geological epochs, when a vast portion of the earth was the bed of 

 ocean, it may be imagined with what force the waves dashed against opjoosing 

 granite rock. The chemical action was already at work, decomposing the 

 crystalline fabric ; while the waves, by their abrasive powers, were grinding- 

 and depositing the dark mass, which was to emerge, in due time, in the 

 form of the slaty shale of the Silui-ian system. The action of the waves 

 and the winds on the stratified rock is still more intelligible ; accordingly, we 

 find that the London clay, embedded over the chalk to the depth of 700 feet, 

 consists of layers of clay, of sand, and of gravel, sometimes marine and some- 

 times fresh-water deposits, as the geologist easily learns by studying its fossil 

 remains, which present every kind of vegetation, from the tropical cocoa-nut 

 and acacia to the walnut-tree ; indicating that the country had passed from a 

 tropical sun at one period to another extreme, when there is every indication 

 that the temperature was that of the frozen zone. There were also shallow 

 seas and lakes at the tropical period, when groves of palm-trees existed, 

 imder whose shade tortoises basked ; and rivers which swarmed with crocodiles, 

 and forests in which the elephant and other tropical animals ranged ; while the 

 group of isolated islands became gradually united into small continents, and 

 the rocks into islands, probably with small inland seas and lakes in their 

 bosom. 



43. As might be expected under these circumstances, when we come to 

 inspect minutely tl'.e soil of any garden or field, it will be found to contain, — 

 1. stones, sand, or gravel, in larger or smaller masses ; 2. a lighter mass of 

 friable soil, crumbling into dust between finger and thumb, and, when put in 



