The Paragenetie Relations of Minerals. 329 



II. Those consisting of silicates with quartz. 



This distinction is of great importance in the study of 

 eruptive rocks, and in relation to the association of some 

 minerals quite essential, indicating likewise the presence or 

 absence of others. The most frequent of such silicates be- 

 long to the genera felsite, pyroxene, amphibole, phengite, 

 astrite, &c. The felsites are especially important both on 

 account of their greater frequency, and because they include 

 trisilicates as well as bisilicates almost entirely destitute of 

 metallic oxides and magnesia, but rich in alumina and 

 alkalies. Together with these there occur in the first class 

 of rocks pyroxenes and amphiboles containing much iron, 

 though scarcely any alumina or alkalies. 



It must not be forgotten that some pyroxenes and amphi- 

 boles are similar in composition, and in such cases may be 

 regarded as dimorphous bodies. Indeed, bronzite and antho- 

 phyllite are identical in composition, corresponding with the 

 formula Fe O Si 3 + 3 Mg 0, 2 Si 3 . It is therefore possible 

 that certain eruptive masses may have yielded either diabase 

 or diorite, according to the prevailing conditions. Mitscher- 

 lich has actually obtained crystals of pyroxene by fusing am- 

 phibole; and G. Rose has shewn that crystals occur, consisting 

 of a nucleus of pyroxene and an envelope of amphibole. In 

 connection with this point, it is worthy of remark that some 

 species of pyroxene occur only in rocks of volcanic, or un- 

 doubtedly igneous origin, such as basalt, lava, &c, while 

 others do not occur ; and in the former case, the pyroxenes 

 are accompanied by astrites, but never by phengites. In such 

 rocks, pyroxene and amphibole are indeed sometimes met 

 with together, but the occurrence of amphibole is then limited 

 to one species — basaltic amphibole, — which is essentially 

 distinct. 



In the rocks consisting of silicates with quartz, for the 

 most part the oldest known rocks, associations of such mine- 

 rals are observed, as, in the present state of science, can 

 scarcely be regarded as of simultaneous formation. When- 

 ever quartz is present as an essential constituent, pyroxenes 

 are not found, with the single exception of spodumene, which 

 although mineralogically belonging to this genus, differs 



