A SKETCH OF GEOLOGY. 7 



and of many different processes through which it has passed during the many 

 ages of its existence. By whatever influences the rocks may have received 

 their present characters, they received their original form and structure by the 

 agency of fire. All the rocks, without exception, passed, in the beginning, 

 through a molten condition, out of which, by subsequent cooling, they re- 

 ceived their first form as solids. But in the course of time another agent 

 appeared and changed many of the existing forms. Water, the powerful 

 opponent of fire, went into action, and by its chemical, as well as mechanical 

 influence, dissolved a large portion of the fire-produced formations, and carried 

 them to distant localities, where, under favorable conditions, they were de- 

 posited as sediments, forming those rocks which are mainly characterized by 

 their arrangement into strata or layers. Again, we find many of these sedi- 

 ment-formations have been subjected to the influence of heat, by which they 

 lost some of their former characteristics. Their crystallization, and the total 

 absence of organic remains, prove the action of heat, while, on the other 

 hand, their stratification, which is generally retained, testifies to their sedi- 

 mentary origin. Thus we will notice a natural division of all the rocks into 

 the following three classes : 



1. Rocks originally formed by fire and not afterwards changed, the igneous 

 rocks. 



2. Rocks formed in water by sediments, the stratified or sedimentary 

 rocks. 



3. Rocks originally formed as sedimentary deposits, but afterwards changed 

 by heat, without losing their stratification, called metamorphic rocks. 



The igneous rocks are generally subdivided into volcanic and plutonic rocks. 

 Their difference is caused by the condition under which the cooling of the 

 molten masses took place. In the volcanic rocks, the molten matter appeared 

 either on the surface of the earth, or at least very near to the same, where the 

 cooling was rapid, and where the forming rocks were not subjected to the 

 heavy pressure of the superimposed strata. 



The plutonic rocks resulted from greatly different conditions. Here the 

 molten masses did not penetrate the surface strata, but remained deep in the 

 interior of the earth, or at least at the bottom of deep oceans, where the cool- 

 ing process was retarded, and where the new formations were compressed by 

 the weight of the overlying layers. 



In the column of strata we generally find the plutonic rock at the bottom. 

 Next above come the metamorphic formations, which are superimposed by 

 the sedimentary and volcanic rocks. This arrangement led the earlier geolo- 

 gists to the belief that the plutonic and metamorphic rocks were older than the 

 others, consequently, they called the lower primary formations, and the upper 

 the secondary formations. The older school of geologists adhered to the 



