72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, 



continue to use that which exists, though under protest. Wholly 

 glassy forms doubtless exist, though prohahly not very abundantly ; 

 but it is sometimes difficult to distinguish these among the pitch- 

 stones and obsidians, of which the majority belong to rocks with, 

 a higher percentage of silica. 



In the diorite group a considerable portion of the plagioclase 

 felspar belongs to one of those which contain a higher proportion of 

 silica than labradorite, though that felspar is often present, and some 

 distinguish a subgroup the lahrador-diorites. In this case, however, 

 much, care will be needed to see that we are not really dealing with 

 a member of the dole rite group, where the augite, by paramorphic or 

 other change, has been altered into hornblende. As in the case of 

 the syenites, we have, then, in the holocrystalline division, diorite. 

 mica-diorite, and augite-diorite, the last being of rare occurrence. For 

 the hemicrystalline division there seems to be a general concurrence 

 in favour of adopting the term porpliyrite, which we can subdivide as 

 in the case of felsite ; and for the hyaline we have the term andesite, 

 which, division is separable into the hornhlende-andesites and the 

 augite-andesites, the latter mineral occurring more frequently 

 is these than in the porphyrites. In these, and especially in the 

 latter, hypersthene has recently been frequently detected in con- 

 siderable quantities, so that its presence requires to be noted by a 

 prefix. Mica is not unfrequent. It is evident that the augite- 

 andesites form a link with the basalts, and the hyperstheniferous 

 augite-andesites approach the norites ; to one or the other many of 

 the rocks called melaphyres really belong. AYhoUy glassy forms no 

 doubt exist, as in the last group. 



As a small outlying group from the syenites we have the minettes, 

 which may be regarded as extreme forms of the mica-syenites, being 

 composed chiefly of orthoclase and mica (commonly biotite), and the 

 kersantites, extreme forms of the mica-diorites. Of both groups 

 hemicrystalline forms, as well as those with a glassy base, exist; but 

 distinctive names have not generally been assigned to them. As a 

 rule, they contain a slightly lower percentage of silica than the 

 normal syenites and diorites. They occur also, so far as I know, in 

 masses of very limited extent, mostly dykes and veins, and are 

 commonly, though not universally, of Pre-Mesozoic age. 



We arrive now at the concluding pair of groups, which dijffer 

 from the last described in these respects : — quartz is present as an 

 essential constituent, and not as an accessory ; in the orthoclase 

 group mica is much more common than hornblende; and mus- 

 covite, or a light-coloured mica of some kind, which has hitherto 

 been rare even as an accessory, becomes sometimes an important 

 constituent. As before, transitional forms between the two groups 

 exist, and probably a plagioclase felspar is never whoUy absent 

 from the orthoclase group. That, however, may be defined, when 

 holocrystalline, as essentially consisting of quartz -j- orthoclase + mica, 

 and is the familiar granite. Some have proposed to restrict this 

 name to those varieties where the mica is muscovite, and to call 

 those with biotite by the name granitite ; the na^jiie pegmatite is 



