376 
VOLCANOES OF THE LOWER OL^D RED SANDSTONE 
BOOK V 
inferred among tlieiu. The beantiful resinous or pitelistone-like rock from 
near Airtlirey Castle has been found by Mr. Watts to be a glassy hyperstliene- 
augite - andesite, since among its phenocrysts of plagioclase, augite and 
hyperstliene both occur. Magnetite is commonly traceable, and apatite may 
be occasionally detected. As the result of decomposition, calcite, chlorite and 
limonite are very generally diffused through the rocks.^ 
(c) The lavas which may be separated as Tkachytes offer no distinctive 
features externally by which they may be distinguished from the andesites. 
Indeed, both groups of rocks appear to be connected by intermediate 
varieties. In the Cheviot Hills some of the lavas are found, on microscopic 
examination, to contain a large admixture of nnstriped porphyritic felspars, 
which can occasionally be I’ecognized as sanidine in Carlsbad twins. The 
groundmass is sometimes a brown glass, but is usually more or less com- 
pletely devitritied, portions of it being inclosed in the large felspars. Chlorite, 
pseudomorphio after augite or enstatite, may be detected, and sometimes a 
brown mica. A specimen of one of these rocks, from a locality to the north- 
west of Mliitton, near Jedburgh, was found by Mr. J. S. Grant Wilson to 
have the following composition : — • 
SiO-j 
AloC);J 
FeaOs 
FeO 
MuO 
ClaO 
MgO 
KqO 
NaoO 
lUO 
Total. 
It.W. of "Whitton Hill, 
Jedburgh (No. 1938) 
Sp. gr. 2 '55. 
62-44 
18-99 
3-35 
1-8 
-25 
1-84 
1-37 
5-02 
2-65 
2-48 
100-19 
1 
(d) Acid rocks such as Felsites and Ehyolites are rare among the 
lavas poured out at the surface during the time of the I,ower Old Eed 
Sandstone. They occur in the Pentland Hills, also near Dolphinton in the 
Biggar district, and in the Ochil Hills near Auchterarder, associated with 
extensive accumulations of felsitic tuffs and breccias. They are usually 
so much decomposed that it is hardly possible to procure fresh specimens 
of them. Some of them display beautiful flow-structure. They appear to 
be generally orthoclase-felsites or orthophyres. Hull, fine-grained to flinty 
in texture, they hardly ever display free quartz, so that they can seldom be 
placed among the typical rhyolites, though in their banded flow-structure 
they often strongly resemble some lithoid varieties of these rocks, especially 
such varieties as that represented in Fig. 9. 
Mr. "Watts, to whom I submitted, for microscopic examination, a 
number of specimens from the Pentland and Ochil Hills, has found them to 
“ consist of a brown felsitic groundmass in which are embedded a generation 
of small stumpy prisms of orthoclase and a set of larger phenocrysts, 
generally consisting of orthoclase and plagioclase in equal proportions. 
Brown mica is usually present and zircons are not uncommon.” The rocks, 
1 Dr. F. H. Hatch supplied notes on microscopic structure which are incorporated in the text, 
together with particulars afterwards furnished by Mr. Watts. 
