BELATTONSHTP OF PLUTONIC AND IGNEOUS BOOKS 59 



amount of devitrification the latter may have undergone owing 

 to its greater geological antiquity. 



The name rhyoUte likewise includes rocks with the structure 

 and composition of the older quartz porphyries, and though 

 intended by Eichthofen to include only certain comparatively 

 modern acid lavas, has been shown by the late Dr. "Williams^ 

 to be applicable to the pre-Cambrian lavas of the South Mountain 

 region of Pennsylvania. These and other names have, however, 

 become too firmly engrafted upon the literature to be too hastily 

 set aside, and may well be retained here. 



The following table will serve to show the relationship, so 

 far as known, which exists between the plutonic rocks and 

 their effusive equivalents of whatever age. Thus the palaeo- 

 volcanie equivalents of the syenites are the quartz-free por- 

 phyries, and the neovolcanic equivalents, the trachytes. The 

 terms, acid, intermediate, and lasic, as used, have reference to 

 the percentage amounts of silica, both free and combined, con- 

 tained by the representatives of the several groups. Eocks which, 

 like some of the peridotites, carry even less than 40% of silica 

 are sometimes spoken of as ultra basic. 



TTurmfSTTOTVIK t\Xt T^T TTTtAWTn 



Effusive oe Volcanic 





Palaeo volcanic 



Neovolcanic 



Acid 



65%~75% 

 SiOs J 



Intermediate 

 65%to65%- 

 SiOa 



Basic 



40% to 66% - 

 Si02 



r' Granites .... 



'Syenites .... 



Nepheline syenites) 



(Poyaites) f 



. Biorites . . . . 



• Gabbros, norites, | 



and diabases j 



Theralites . . . 



Peridotites . . . 

 Pyroxenites . . . 

 (Not known) . . 

 (Not known) . . 

 . (Not known) . . 



Quartz porphyries . . 



Quartz-jEree porphyries 

 Phonolites 



Porphyrites .... 



Melaphyrs and angite) 



porphyrites ( 



(Not known). . . . 



Picrite porphyrites 

 (Not known) . . . 

 (Not known) . . . 

 (Not known) . . . 

 (Not known) . . . 



Liparites (rhyolites) 



Trachytes 



Phonolites 



Andesites 



Basalts 



jThephrites and 

 ( basanites 

 Limburgites 

 Augitites 

 Leucite rocks 

 Nepheline rocks 

 Melilite rocks 



^Am. Jour of Science, Vol. XLIV, p. 482, 1892. 



