CHAP. XVII 
MATERIALS ERUPTED— ACID ROCKS 
277 
when they undergo weathering, pass into the varieties formerly comprised 
under the name clay stone. . 
The only nodular felsite of this age which I have met with is that 
of Lough (luitane among the “Dingle Beds,” near Killarney, to wine 
reference will be made in later pages. ,1,1, i + 
2 . Intrmim Bosses, Sills and Dykes.— mule the interbedded lava-sheets 
are mainly andesites, the intrusive rocks are generally more acid, and inost 
of them may be grouped under the convenient liead of felsites. borne 
intrusive andesites, and even more basic rocks, do indeed occur in dykes anc 
sills, as well as also filling vents. But the rule remains of general applica- 
tion over the whole country that the materials which have consolidated in 
the volcanic orifices of the Old Ked Sandstone, or have been thrust among 
the rocks in dykes, bosses or sills, are decidedly acid. In this series of 
rocks a greater range of types may be traced than among the extrusive 
lavas. At the one end we find true 
ffi-anites or granitites, as in the in- 
JclVclb. -nhu Uiio viio o . 11 . 1 " 1 
trusive bosses of Spango Water and of Galloway, which, lor reasons which 
I will afterwards adduce, may with some probability be assigned to the 
volcanic history of the Lower Old Ked Sandstone period. Among t^he 
bosses, many of wliich probably mark the positions of eruptive vents, ortho- 
phyres are especially prominent. These rocks Irequently contain no 
miL, but, on the other l.and, they sometimes show abundant cpiartz in 
their groiindmass. The aiigite-graiiitite of the Cheviot Hills, so well 
described liy Mr. Teall, has invaded the bedded andesites of that legioii. 
A similar rock has been noticed by my brother. Prof. James Geikie 
associated with the Lower Old Ked Sandstone volcanic rocks of the east of 
Ayrsliire A remarkable petrographical variety has been mapped bj 1. 
B N. Peach rising as a small boss through the lower part of the peat 
lava-sheets of the Ochil Hills, above Tillicoultry. It is a granoplp'i’ic 
quartz-diorite, which, under the microscope, is seen to be composed of short, 
thick -set prisms of plagioclase, with abundant granophyric quartz, a 
pleochroic hypersthene,. and needles of ap^ite. Sometimes the pyroxene is 
replaced by green chloritic pseudomorphs. 
At the other end of the series come the felsites, quartz-porphyries, inica- 
porphyrites, ininettes, vogesites, “ hornstones ” and “ claystones (or decayed 
felsites), which have a close-grained texture, often with poipiyritic fe sp , 
quartz or black mica, generally a whitish, pale buff, orange, pink 01 puip 1 
grey colour, and a specific gravity of about 2 55 . 
Though I class these rocks as intrusive, I am not prepaml to asspT 
that in none of the instances where they occur as sheets may they possibly 
have been erupted at the surface as lavas. In one or two casp the .evidence 
either way is doubtful, but as the great majority of the acid rocks can be 
1 Geol. May. for 1883, pp. 100. 145, 252 ; and British Petrography, pp. 272, 278. 
s Thf hUn°ive^“*porphyTy” of Lintratlien in Forfarsliive (wliicli may be younger tlian 
the Old Red Sandstonef is alright red rock with porphyritic felspar, quartz, white mica an a 
very singular black mica (Mr. Teall’s British Petrography, p. 286). 
