12 NEW YORK STATE MUSEUM 



Section i 



GENERAL FEATURES OF ROCKS AND THEIR COMMER- 

 CIAL ADAPTABILITY 



THE ORIGIN AND CLASSIFICATION OF ROCKS 



Rocks may be defined in simplest 'terms as mineral aggregates. 

 To this definition there may be added also the quality of solidity, 

 an inseparable characteristic perhaps in the popular mind, though 

 not essential from the standpoint of the geologist. These aggre- 

 gates are made up of a variety of minerals, either singly or in 

 mechanical mixture. They also differ among themselves in their 

 structural features, in the manner in which the minerals are as- 

 sembled and held together, that is, their textures, and of course 

 according to origin. . 



The consideration of origin is the most important for the classifi- 

 cation of rocks in the first instance. On that basis they may all 

 be divided into two general groups : ( i ) the igneous rocks, which 

 include all that have consolidated from a molten state and (2) 

 the sedimentary rocks, inclusive of all that have been deposited by 

 water, either in a state of suspension (mechanical action) or solu- 

 tion (chemical action). To the latter may be added also the small 

 class of wind-laid or eolian deposits which are closely allied with 

 the mechanical sediments in their structure and features of occur- 

 rence. 



To these groups which embrace all rocks from the standpoint of 

 origin, it is custoimiary tO' add a third group of coordinate rank in 

 the classification, or (3) the metamorphic rocks. This group in- 

 cludes those members of either igneous or sedimentary derivation 

 that have undergone great changes which involve a physical re- 

 arrangement and also at times a chemical transformation of the 

 components with the development of a new set of minerals. 



There is naturally no .sharp line of division between the meta- 

 morphic and the other groups ; on the other hand, the process of 

 change may be followed in many cases through all the stages from 

 the one to the other, as from an unaltered sediment like clay 

 through shale and slate to hard and thoroughly crystallized schist 

 or gneiss. It is the general practice, however, to place only the 

 more completely changed types in the metamorphic class, and 

 especially those whose origin may not readily be discovered. 



