CHAPTER IV. 



PETROGRAPHY. 



THE investigation of the rocks which compose our earth's 

 crust has always been conducted along two directions of study : 



(1) the investigation of the mineralogical and chemical 

 composition, the structure of rocks, their mode of occurrence ; 



(2) the investigation of their mode of origin. 



The systematic arrangement and the morphology of rock- 

 varieties has been constructed mainly upon a mineralogical 

 basis ; the questions concerning the origin of rock-varieties 

 have been handled more from the geological and chemical 

 side. A distinction between massive eruptive rocks and 

 stratified deposits \vas early recognised in petrographical 

 literature. Mutton's was the genius which first differentiated 

 clearly between plutonic, volcanic, and sedimentary rocks in 

 point of origin ; while Werner, too biassed by Neptunistic 

 doctrines to perceive the fundamental truths which Hutton 

 had taught, nevertheless accomplished the task of erecting a 

 systematic classification of rocks upon mineralogical con- 

 siderations. 



During the first half of the nineteenth century, all petro- 

 graphical works followed Werner's system. His determina- 

 tion of rocks as simple or composite occurs in most of the later 

 attempts at classification, and also his fundamental principle 

 of differentiating the essential and the accessory minerals in 

 mixed rocks has been continued to the present day. 



Brongniart had in 1813, in his table of Composite Rocks, 

 assigned great importance to the structural relations, and dis- 

 tinguished accordingly three chief classes : i, the " isomerites," 

 or granitoid varieties of rock, in which the individual elements 

 are united only by crystalline aggregation, and there is 

 no finer matrix, e.g., granite, syenite, protogine ; 2, the 

 "anisomerites," or porphyritic and hemicrystalline varieties, 



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