THE TEXTURE OF IGNEOUS ROCKS 



WHITMAN CROSS, J. P. IDDINGS, L. V. PIRSSON, H. S. WASHINGTON 



The vagueness that characterizes descriptions and conceptions 

 of textures of igneous rocks is due in part to a lack of systematic 

 treatment of the subject by petrographers, and a consequent inexact- 

 ness in the terminology by which definite conceptions may be expres- 

 sed. It is also due to the intricacies of the textures to be described 

 and to their diversity. 



The first may be remedied by more systematic consideration of 

 the fundamental factors that make up rock textures, and by the 

 introduction of more definite descriptive terms. The second diffi- 

 culty can be met by more accurate and detailed descriptions of the 

 complex fabrics, and by the recognition of the more frequently 

 recurring ones as types for reference. 



In considering the fundamental factors constituting rock texture 

 it is necessary to begin with the simpler, elemental factors, and also 

 the simpler fabrics; afterwards taking up the more complex and 

 intricate fabrics. This is not the order of the more common first, 

 and subsequently the less frequent fabrics. On the contrary, many 

 of the commonest fabrics are the most intricate and the most difficult 

 to describe. This is probably a reason why the subject has never 

 been satisfactorily developed. A systematic treatment must com- 

 mence with more or less abstract, general, principles, which eventually 

 may be applied to concrete cases. It is hoped that the following 

 discussion may advance this part of petrography. 



Fundamental factors constituting texture in igneous rocks. — 

 Defining texture as 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, 

 it has already been said^ that these features fall into three categories: 



1. Crystallinity — the degree of crystallization. 



2. Granularity — the magnitude of the crystals. 



I This Journal, Vol. X (1902), p. 611. 



692 



