INTRODUCTION. 



fpHE rocks of which the earth is composed, or which fall on the earth's surface 

 J_ from outside space form the subject matter of petrography. It is 

 extremely difficult to frame any definition of the term rock, to which excep- 

 tion may not be taken. In its widest sense it may be said, without risk of 

 serious error, to include every solid mass (1) of considerable size which forms 

 an integral portion of the earth or of some other cosmic body, and which 

 cannot be referred to one crystalline or organic individual. As a general rule 

 rocks are aggregates, composed of individual substances which owe their solid 

 form either to crystalline or organic activity. In the present work we 

 shall limit our attention, for the most part, to those masses which are 

 important as constituent portions of the earth's crust, and which show, there- 

 fore, a considerable amount of uniformity when traced over extensive areas. 

 Meteorites or extra-terrestrial rocks, and the contents of mineral veins, lodes 

 and drusy cavities will not be referred to at any length. 



At the outset it is necessary to remark that petrography is a branch of 

 geology, and not merely a department of mineralogy. Rocks are, it is true, 

 for the most part mineral aggregates, and much of the work of the petro- 

 grapher is therefore of a mineralogical character. His principal attention, 

 however, is directed not to the study of the rock-forming minerals as indi- 

 viduals, but to their mutual relations in the rocks themselves, and to the 

 behaviour of the rock masses as constituent portions of the earth's crust. The 

 minerals, quartz and felspar, occur as constituents of a variety of granite, 

 and also of arkose, a rock resulting from the disintegration of granite, and 

 the deposition of the constituents under the influence of surface agencies. 

 A petrographical classification, based on mineralogical composition alone, 

 would therefore place granite and arkose in the same group, and thus ignore 

 a most important and well established fact in the history of the latter rock. 



The above illustration serves to emphasise another most important fact 

 with reference to the science of petrography. Rocks may be studied from 

 two more or less distinct points of view the descriptive and the setiological. 

 We may set to work to describe them to ascertain and record every possible fact 

 with regard to their composition, structure and mode of occurrence or we may 

 endeavour to trace the succession of events which has culminated in the state 

 of things we actually observe. Descriptive petrography deals with : 



(1) The physical and chemical characters of the rock as a whole. 



(2) Its mineralogical composition. 



(3) The forms and mutual relations of the constituent parts. 



(1) The expression " solid mass " must not be interpreted too literally. Interspaces frequently 

 occur between the constituent parts of certain rocks, and gas and fluid cavities are often found 

 in the individual minerals of which rocks are composed. Again, the term rock is also applied to 

 loosely coherent masses of gravel, sand and clay. 



