THE CLASSIFICATION OF IGNEOUS ROCKS. 473 



who adopted this view, whether separating composite rocks 

 from simple minerals or not, were content, for the most 

 part, to group the rocks in accordance with the dominant 

 mineral of each. At that time there were, however, formid- 

 able obstacles in the way of any mineralogical classification 

 of rocks. In the first place, the minerals themselves were 

 very imperfectly known, and their mutual relations scarcely 

 understood at all. A grouping of minerals according to 

 their " external characters " could not lead to a rational 

 classification of rocks upon a mineralogical basis. This 

 difficulty naturally disappeared with the advance of mineral- 

 ogy, especially with the importance attached by Ha'uy and 

 his school to crystallography, and the chemical investiga- 

 tions of Berzelius and others. A more serious obstacle, in 

 what we may term the pre-microscopic days of Petrology, 

 was that the fine-textured aggregates and the compact 

 ground-mass of many porphyritic rocks defied mineralogical 

 analysis. The writers who, in the earlier decades of this 

 century, advocated a mineralogical as opposed to the so- 

 called geological system of classification, were accustomed 

 to divide all rocks into homogeneous and heterogeneous, 

 including under the former head such substances as felsite 

 (" compact felspar") and basalt, which were not resolved by 

 the methods then in use. Cordier's demonstration, in 

 18 1 5, of the composite nature of the compact ground-mass 

 sufficed to shake but not to destroy this distinction. Thus 

 we find Brongniart, twelve years later, subdividing his 

 '' homogeneous " rocks into two orders — " phanerogenes " 

 and " adelogenes " ; the former being simple rocks consist- 

 ing in each case of one known mineral ; the latter being 

 formed "wholly or in part by a mechanical union of mineral 

 particles," which he makes no attempt to specify. Such 

 rocks as basalt are included in this latter order, while even 

 in the former we find, e.g., lherzolite under the head of 

 pyroxene. 



The microscope has now almost completely removed 

 the difficulties felt by the earlier petrologists, although the 

 ground-mass of many acid rocks — the " petrosilex " which 

 baffled Cordier — has furnished material for controversy down 



