80 WHITMAN CROSS 



rocks are the materials which make up the crust of the earth, 

 and to distinguish them from minerals it is pointed out that a 

 rock is a geological body — a geological unit. But it is the unit 

 of material or substance, and must be clearly distinguished 

 from the geological unit of form or mass. It is the substance of 

 the geological body, but the two conceptions are not coex- 

 tensive. Chemical and mineralogical composition and structure 

 are the chief characters of rocks as concrete objects, and it is 

 well known that neither a stratum of sedimentary rock, nor a 

 dike of igneous rock, is necessarily of the same composition or 

 structure throughout. The rock unit cannot be that of the geo- 

 logical body as long as this is true. The rock unit is simply that 

 which the systematic scheme of rock classification finds most 

 desirable and practicable. 



It is universally recognized that rocks not only have many 

 relationships, when viewed from the geological standpoint, but 

 that as objects they are extremely complex and variable in char- 

 acter. Lossen has said that the property of transition in all 

 directions is an essential characteristic of the rock. It is quite 

 possible that some who have struggled with systematic problems 

 may be inclined to define the rock as the most variable and 

 indefinite thing in nature ! Yet rocks must be classified and 

 named according to some system, and the task is none the less 

 interesting or important because of the difficulties involved. The 

 best system, that most nearly natural and logical, most uniform 

 and stable, must ultimately prevail. 



I understand that branch of geology which is concerned with 

 rocks in all their aspects to be petrology — a treatise on rocks — 

 and the narrower systematic, descriptive science of rocks as con- 

 crete objects, the basis for their specific nomenclature, to be 

 petrography. This usage has now become so current in this 

 country that discussion seems unnecessary. 



Let us pass in review some of the different aspects of rocks 

 which must be considered by the geologist, and at the same 

 time we shall outline the field embraced by petrology, (i) There 

 is the rock itself — an object of variable and complex character. 



