NORTH AMERICAN GEOLOGY. 



CHAPTER I. 



DEFINITIONS AND LAWS OR OEXDLOQY. 



1. GEOLOGY is the science which comprehends the structure of the earth and 

 investigates its history. It does not extend to the beginning, nor throw any light 

 upon the astronomical theory that the world was, at one time, in a gaseous state, 

 and later in a condition of fluidity. It commences at the most ancient rocks found 

 upon the surface of the earth. These had their origin, in sedimentary deposition, 

 at the bottom of an ocean. The world was then as large as it is now, and beyond 

 the fact that these rocks were once merely sedimentary layers at the bed of a 

 sea, the previous history of the earth is unknown, and all prior time is impenetrable 

 darkness. Geology commences at the lowest discovered rocks, and investigates the 

 overlying strata, the changes that have taken place, the lapse of time, and the de- 

 velopment of organic life, to the present moment. If the strata of rocks on the surface 

 of the earth were horizontal, the science would extend over a short period of time, 

 and might be learned as rapidly as we progress in zoology, anatomy, or other 

 branches of Natural History ; but the rocks are inclined at various angles, and form 

 synclinal troughs and anticlinal ridges, and expose, in the order of sedimentary de- 

 position, at the maximum more than forty miles in thickness. Mountain regions 

 rarely afford so good opportunities for the study of Geology as a country unbroken, 

 except by the exposures in stone-quarries and the banks of streams. In some States 

 the dip of the strata is quite uniform for a hundred miles or more, without any folds 

 or flexures. It is in these areas the student will find the most inviting fields for the 

 study of the science. 



2. The laws of the science have been ascertained, from observation and in- 

 vestigation of the changes now taking place, from a knowledge of those which have 

 occurred within the historical period, from the evidence of change in more remote 

 ages, from the study of the skeletons and harder parts of animals and plants, and 

 the process of infiltration of mineral matter into these organisms, which fills up the 

 cavities and produces petrifactions, and from the study and determination of the 

 characters of the petrifactions found in the rocks of nearly all ages. Neither plants 

 nor animals turn to stone; flesh can not petrify. When a body is sufficiently firm 

 to preserve its form until water, holding lime or silica in chemical solution, can 

 penetrate the cavities, saturate it, and deposit the stony matter as the organism 

 decays, we have a fossil or petrifaction. The laws of nature are uniform in their 

 operation. The diversified character of the rocks has resulted from general causes, 

 and the uplifting and inclination of sediments did not occur in one period of time, 

 but are distributed through and belong to all geological ages. We do not assume 



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