GEOLOGY. 



life is also busy in the formation of rocks. Away 

 in the warmer seas of the Pacific, lives the cora 

 insect or zoophyte, the skeletons of which compose 

 the remarkable coral reefs that form the chief part 

 of the numerous isles that stud that greatest o 

 seas. These reefs extend thousands of miles, in 

 broad barriers, over which the wild waves dash 

 or in detached groups that gradually gather mate- 

 rial round them, and form new islands. In the 

 rocks, we also find the remains of like corals, 

 standing where they grew, or drifted away, and 

 -appearing as extensive formations of limestone. 



Again, the bottom of the sea is covered with 

 .accumulations of minute shell-fish, of great depth 

 and extending over wide areas, as is proved every 

 -day by soundings with the lead. The old rocks 

 also exhibit strata identical in composition with 

 these microscopic shells ; some limestones and 

 -chalks, for example, being composed of millions to 

 the square inch of perfect bivalve shells. Again, 

 the sea-bottom contains beds of shell-fish, of differ- 

 ent kinds, and of great extent and thickness. 

 If these were to die, and be subjected to sufficient 

 pressure, they would form a rock, exactly like the 

 shell limestones so common in our rock-formations, 

 and so valuable in agriculture and building. 



Then we possess the remains of ancient forests 

 in our great mosses ; and luxuriant growths of 

 swampy plants and impenetrable jungles in the 

 mud islands of the deltas of our great rivers in the 



tropics. These, submerged and acted on by heat 

 and pressure for ages, would become coal, similar 



to what we daily use for fueL Thus, organic life 



of all kinds is everywhere busy in forming rock- 

 masses, similar in character and appearance to 



those presented to our investigation in limestone, 



chalk, and coaL 



AGENCIES IN THE ALTERATION OF ROCKS. 



DISTURBING AGENTS. Stratified rocks in their 

 natural state would be more or less horizontal and 

 continuous. How, then, are we to account for the 

 tiltings, upheavals, faults, and various dislocations 

 so prevalent among the strata? Igneous forces 

 furnish the solution. The whole globe is subject 

 to convulsive movements produced by internal 

 forces, which are seen in earthquakes, and by 

 which the ground is torn into fissures, and the 

 solid crust made to move in mighty undulations, 

 that destroy and swallow great cities. Extensive 

 tracts are also sometimes suddenly raised or 

 depressed. Sometimes, too, great yawning craters 

 open where previously volcanic movement was 

 unknown, and continue for a time in active erup- 

 tion. In these upheavals and subsidences, sudden 

 or gradual, of extensive tracts, we see the causes 

 -at work of the dislocations of the rocks of former 

 times, and of the elevations and depressions that 

 occurred throughout the geologic eras. 



Again, we know that in order to the deposition 

 of strata of any thickness, the sea-bottom must 

 have gradually subsided : does any such gradual 

 subsidence take place at the present time ? It is 

 ascertained, from extended observations, that on 

 the northern shores of the Baltic, for instance, 

 there has been a gradual rise at the rate of 4 feet 

 in a century ; and, in South America, a rise of 85 

 feet during the human period, at Valparaiso, 

 of 19 feet in 220 years ; while over all the world, 

 and even round our own coasts, ancient sea- 



beaches may be seen at various elevations, mark- 

 ing former sea-levels. On the other hand, the 

 south coast of Sweden, the coast of Greenland 

 over 600 miles, and parts of South America for 

 the last 300 years, have been slowly sinking ; nor 

 are the British shores free from such oscillations. 



Thus, again, we see that existing causes perfectly 

 explain the gradual subsidences and upheavals 

 necessary to the formation of the rocks and to 

 their subsequent elevation into dry land. 



DISINTEGRATING AGENTS. Every stratified 

 rock in the immense thickness of the crust of the 

 globe has been formed of the debris of pre-existing 

 formations, that have been ground down and held 

 in suspension till deposited in the layers after- 

 wards hardened into rock. Whence, then, this 

 immense accumulation of matter, and what the 

 disintegrating agents ? 



I. Atmospheric Agency. The atmosphere, by 

 its chemical action, and by the combined effects 

 of alternate heat and cold, wetness and dryness, is 

 continually crumbling down all exposed surfaces, 

 forming new soil, and thus increasing the earthy 

 covering of the globe. The wind, also, has an 

 incredible power of drifting and heaping up sand- 

 hills along the shore as in the county of Elgin, 

 where an ancient barony has been entirely reduced 

 to a desert through this means and of raising 

 the waves of the sea, and wearing the rocks 

 through the mighty force of its swooping billows. 

 Frost, too, is one of the quietest but most powerful 

 disintegrating agents; for when water has per- 

 colated a mass of rock, the act of freezing exerts 

 a great expansive force, which cracks the rock. 

 But frost can work on a grander scale, for to its 

 agency is due the existence of avalanches, glaciers, 

 and icebergs ; which, whether sweeping with 

 overwhelming convulsion, or crawling down the 

 mountain side, or floating and grating on the ocean 

 floor, continually and with terrible effect, wear 

 down or dash to pieces every rock that obstructs 

 their irresistible course. 



2. Aqueous Agency. The most extensive aque- 

 ous agent is rain, which wears, softens, percolates, 

 and gradually wastes away the rocks on which 

 it falls. Rain-water also gathers under the 

 ground in large cavities, where springs are formed, 

 which dissolve the interior rocks, and, bursting 

 out, deposit their solutions of lime, iron, sulphur, 

 soda, flint, and bitumen. One of the most power- 

 : ul degrading agents is, of course, the sea, which, 

 as it beats on its rocky shores, wears, rolls, 

 and grinds to powdery sand the flintiest rocks, 

 and presents, as monuments of its mighty power 

 of waste, those lofty cliffs that guard its shores. 

 But more powerful, but less obvious agents of 

 destruction than the sea, are seen in the many 

 streams that everywhere traverse the land on their 

 way to this boundless reservoir. The power of 

 rivers in excavating and wearing away the surface 

 of the globe, is much greater than at first thought 

 might be supposed. Most river-valleys, however 

 deep, have been mainly worn down by river-action, 

 extending over immense periods of time. When 

 we contemplate the mighty valleys, inclosed by 

 owering peaks capped with eternal snows, that lie 

 hid amidst the mountain solitudes of the Alps, the 

 Andes, or the Himalaya, we may well be aston- 

 shed at such a statement But that these huge 

 excavations have been mainly produced by the 

 combined action of air, frost, rain, and river, has 



