566 CROSS, IDDINGS, P IRS SON, WASHINGTON 



which will produce the most natural and stable as well as com- 

 prehensive and elastic arrangement. 



Of geological relations, the mode of origin is now universally- 

 recognized as the first principle to apply to the sub-kingdom of 

 rocks, to secure the grand divisions of which the igneous rocks 

 are one. The further application of geological mode of origin 

 in subdivision of igneous rocks involves the use of theoretical 

 considerations and produces instability of system. 



The relations of geological occurrence have been used in the 

 arrangement of igneous rocks, but unsuccessfully. Geological 

 age cannot now be used without violating the known fact that 

 rocks of many ages are identical in their material qualities. 



Some systems of classification now in use are based on sup- 

 posed relationships between the material characters of igneous 

 rocks and their modes of occurrence as geological bodies, which 

 are only partially in accord with facts, and which therefore intro- 

 duce serious weakness into the foundation of the systems. There 

 are no particular kinds of rocks that invariably characterize geo- 

 logical bodies of special shapes, such as stocks, laccoliths, dikes, 

 etc. Nor is there any specific texture that indicates the depth 

 beneath the surface of the earth at which a rock has crystallized. 

 While there is unquestionably a relation between the texture and 

 mineral composition developed in a given magma and its physical 

 environment during eruption and intrusion and at the time of its 

 solidification, this relationship is so intricate, and the possibili- 

 ties of environment so manifold, that it cannot be made a basis 

 for classification. 



The effort to classify rocks on a basis of their genetic rela- 

 tionships by grouping them in such a manner as to express the 

 fact that all the rocks of a particular center of eruptive action 

 are the differentiates of some common parent magma introduces 

 the utmost complexity, because each group presents a particular 

 set of relations, and it becomes necessary to recognize almost as 

 many groups as there are known centers of eruption. But as 

 already stated, the members of several groups may resemble one 

 another so closely as to be capable of the same definition and 



