270 C03M03. 



If we cast a glance on the geographical distribution of 

 rocks, and their relations in space, in that portion of the earth's 

 crust which is accessible to us, we shall find that the most 

 universally distributed chemical substance. is silicic acid, gen- 

 erally in a variously-colored and opaque form. Next to solid 

 silicic acid we must reckon carbonate of lirne, and then the 

 combinations of silicic acid with alumina, potash, and soda, 

 with lime, magnesia, and oxyd of iron. 



The substances which we designate as rocks are determin- 

 ate associations of a small number of minerals, in which some 

 combine parasitically, as it were, with others, but only under 

 definite relations ; thus, for instance, although quartz (silica), 

 feldspar, and mica are the principal constituents of granite, 

 these minerals also occur, either individually or collectively, 

 in many other formations. By way of illustrating how the 

 quantitative relations of one feldspathic rock differ from anoth- 

 er, richer in mica than the former, I would mention that, ac- 

 cording to Mitscherlich, three times more alumina and one 

 third more silica than that possessed by feldspar, give the con- 

 stituents that enter into the composition of mica. Potash is 

 contained in both a substance whose existence in many kinds 

 of rocks is probably antecedent to the dawn of vegetation on 

 the earth's surface. 



The order of succession, and the relative age of the different 

 formations, maybe recognized by the superposition of the sed- 

 imentary, metamorphic, and congioaierate strata ; by the na- 

 ture of the formations traversed by the erupted masses, and 

 with the greatest certainty by the presence of organic re- 

 mains and the differences of their structure. The application 

 of botanical and zoological evidence to determine the relative 

 age of rocks this chronornetry of the earth's surface, which 

 was already present to the lofty mind of Hooke indicates ono 

 of the most glorious epochs of modern geognosy, which has 

 finally, on the Continent at least, been emancipated from the 

 sway of Semitic doctrines. Palaxmtological investigations 

 have imparted a vivifying breath of grace and diversity to thb 

 science of the solid structure of the earth. 



The fossiliferous strata contain, entombed within them, the 

 floras and faunas of by-gons ages. We ascend the stream of 

 time, as in our study of the relations of superposition we de- 

 scend deeper and deeper through the different strata, in which 

 lies revealed before us a past world of animal and vegetable 

 life. Far-extending disturbances, the elevation of great mount- 

 ain chains, whos^ relative ages we are able to define, attest the 



