122 Proceedings of the Royal Irish Avadetny. 



The gneissyte and granitoid-felstone that occur in bedded masses 

 are more or less hard to determine, as some, especially among the 

 gneiss, are very similar to many of the metamorphosed sandstones 

 and grits. They, however, generally weather with a white criist some- 

 what like a felstone, which weathering among the metamorphosed 

 sedimentary rocks is unusual only on felsitytes and felsitic-quartzytes. 

 Moreover, in some cases a rock that apparently is bedded, when care- 

 fully traced will be found to cross or intrude into some of the associated 

 bedded rocks. 



Geanitic Rocks. — The granitic rocks may be highly siliceous, or 

 more or less basic ; both of these groups contain rocks that are intru- 

 sive, the latter others that have been formed in situ by extreme metamor- 

 phic action. Previous to entering into the description of the typical 

 granites, the elvanytes or quartz-porphyrites should be enumerated 

 and described, as they are the granitic-rocks next in order to the 

 plutonic-rocks. 



Elvanyte or quartz porphyry. — These passage-rocks between the plu- 

 tonic-rocks and the typical granites are always more or less granitoid. 

 They seem to belong to the granitic-rocks, as they are hypogene, and 

 consequently never are associated with tuff or any other such mechani- 

 cally formed accompaniment. Nevertheless, in part they are allied to 

 the plutonic-rocks as they graduate into them ; moreover, at the 

 extreme margin of wide dykes, also in small dykes, branching fi'om a 

 large one, a rock often occurs that in aspect is undistinguishable from 

 a plutonic-rock. These compact portions at the walls of dykes are never 

 more than a few inches in thickness.* 



Elvanytes range from highly siliceous to basic, f according as they are 

 the passage-rocks from felstone or whinstone to granite. 



Elvanyte has a more or less crystalline felsitic or felspathic 

 base, usually with globules, bleds, or ciystals of quartz, crystals 

 of felspar, and flakes of mica or ripidolite or crystals of amphibole ; 

 pyrite also is often present, beside other minerals, generally as 

 accessories, but sometimes locally as essentials. In all elvanytes 

 some of the quartz seems to have crystallized out first, but some- 

 times only sparingly. In the highly siliceous varieties this quartz 

 is characteristic of the rocks, while in the more basic varieties 

 it is often scarcely perceptible. Scheerer excludes from his tnie 

 granites rocks in which part of the quartz crystallized out before the 

 other constituents, as will be seen from the following epitome of this 

 eminent geologist's definition for granite : — " The crystals of felsjjar, and 

 others not containing water, crystallized out first ; the mica, which con- 



* The compact portion is called by Cotta the base or mother-rock. Rock classi- 

 fication, 1. c, p. 214. 



t There are granitoid-basic-rocks vej- y similar in appearance to some of the pre- 

 viously mentioned varieties of hornblende-rock ; some of these probably belong to this 

 gronp ; however, in the country to which this essay refers this could not be proved. 

 There are, however, as will be hereafter stated, rocks that might be called syenyte, 

 which belong to this group. 



