ROCK CLASSIFICATION 109 



the whole ; in others the chemical and mineral composition 

 varies. 1 These variations may be slight and within what may 

 be considered the limits of variation for a given class of rocks, 

 or they may be so great as to exceed these limits, and different 

 parts of one continuous rock-mass may have characters belong- 

 ing to different classes of rocks in the usual sense. The same 

 variations and relations exist between different parts of some 

 rock-masses as those just mentioned as existing between some 

 rock members of various genetic series of rocks, and for exactly 

 the same reason. Hence, definitions of rocks based on the essen- 

 tial, material characters — composition and texture — cannot be 

 made so as to discriminate always between various rock-masses. 



Rock-masses of this kind are geological bodies, in that they 

 form integral parts of the crust of the earth, such as lava 

 streams or sheets, dikes, laccoliths, stocks, etc. Rock-masses 

 or rock-bodies, in distinction from rocks as considered in this 

 discussion, can not be grouped in classes on the same basis as 

 those on which the rocks can. They may be classified accord- 

 ing to their form or dimensions, or their mode of occurrence, or 

 according to some general idea of composition, as homogeneity 

 and heterogeneity. 



A knowledge of the characteristics of all rock-bodies as such 

 is necessary to a proper understanding of the mode of occur- 

 rence of igneous rocks, and is essential to any comprehensive 

 treatment of the science of petrology. 



The nature of genetic series of rocks. — The character of series of 

 rocks that occur associated in any region so as to be consid- 

 ered as genetically related, differ widely in different cases. 

 Aside from the fact that all those in one region may have solidi- 

 fied within the crust of the earth, while those in another region 

 may have reached the earth's surface and have consolidated at 

 the surface and also within the crust, there are the subsequent 

 geological events that have transpired in each case since solidi- 

 fication whereby the present exposure of the rocks has been 



'Iddings, J. P., Genetic Relationships among Igneous Rocks, Jour. Geol., 

 Vol. I, Chicago, 1893, pp. 833-844. 



