6 10 CROSS, IDDINGS, PIRSSON, WASHINGTON 



is somewhat like that of the rock in question and whose actual 

 mineral components can all be determined. 



Texture. 



The terms structure and texture are commonly used in petrog- 

 raphy as synonyms, although efforts have been made to dis- 

 criminate between them. It seems to us desirable to employ 

 them in different senses, and we propose to limit the use of the 

 term structure to those large features of rock bodies which are 

 known as columnar structure, spheroidal parting, platy parting, 

 bedding, brecciation and others. And we use texture for the 

 material features of rocks exhibited by the mineral components 

 and by the groundmass of dense or glassy rocks, whether they 

 are viewed megascopically or microscopically. These features 

 are the expression of the mutual relations of the mineral par- 

 ticles or vitreous portions of rocks. 



The texture of igneous rocks is one of their most variable 

 characters. It depends in very large degree upon extraneous 

 conditions influencing the consolidation of the magma and in 

 far less degree upon certain relationships between the forms of 

 the rock-making minerals. The factors influencing it are many. 

 Nearly the whole range of textures may be developed in rocks 

 from a single magma consolidating under different conditions. 

 Consequently the use of texture as a primary factor in the classi- 

 fication of igneous rocks can only result in the wide separation 

 of things alike in chemical and mineral characters, which, on the 

 grounds discussed in the preceding pages, are believed to be of 

 greatest systematic importance. 



Its use as a prominent factor in classifications has been in 

 most cases coupled with assumptions or assertions known to be 

 by no means strictly in accordance with facts. When such a use 

 of texture is to be made, it is necessary to assume that all rocks 

 possess some one of the few textures at present employed in 

 classification, such as the granular, porphyritic, trachytoid, 

 glassy, etc. This further requires that these textures be so 

 broadly defined that they are robbed of much of their natural 



