274 VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
1 
SiOo 
AtjOs 
Fe-jOs 
FeO 
CaO 
MgO 
KaO 
NaoO 
HoO 
P-..05 
COd 
Carnethy Hill* 
51-16 
22-27 
2-94 
4-02 
5-61 
3-46 
2-42 
2-58 
3-42 
0-48 
1-28 
Biiiselaw. 
Sp. grav. - 
2-80. 
'Soluble in 
HCl 
Insoluble in 
... 
1-30 
1-53 
1-14 
2-43 
0-98 
0-.32 
^ ditto 
62-00 
17-46 
7-85 
6-80 
1-06 
1-66 
4-17 
2-68 
: Warklaw 
1 Hill. 
' Soluble in 
HCl 
... 
5-23 
7-32 
7-88 
3-65 
0-12 
5-01 
Sp. grav. 
2-77. 
Insoluble in 
ditto 
47’77 
13-08 
0-84 
4-07 
0-30 
1-17 
2-30 
2-48 
Th<5re was a trace of iiiaiigaiious oxide in this speci)iieii. 
(5) The Andesites, or, as they were formerly called, Porpiiyrites, 
which constitute by far the largest proiiortioii of the lavas, have a 
characteristic but limited range ot lithological varieties. The prevailing 
type presents a close-grained, rather dull texture, and a colour varying froin 
pinkish grey, through many shades of green and lirown, to purplish red, 
wliich last is, on the whole, the predominant hue. Minute lath-shaped 
felspars may frequently he detected with the naked eye on fresli surfaces, 
while scattered crystals, which are generally hamiatitic pseudornorphs after- 
some pyroxene, occasionally after horiddende or mica, may often be 
observed. The usual porphyritic constituents are plagioclase felspars, 
occasionally in abundant tabular crystals measuring half an inch or more 
across, also one or more pyroxenes (augite, enstatite), and sometimes Irrown 
or black mica. Where large felspar-crystals occur in a compact green 
matrix, the rock assumes a resemblance to the verde. antiq'm of the ancients.^ 
One of the Cheviot andesites lying at the bottom of the series is distin- 
guished by its large and abundant plates of black mica.'^ 
The texture of the andesites occasionally becomes faintly resinous, wliere 
a considerable proportion of glass still remains undevitrified, as in the well- 
known varieties from the Clieviot Hills, and in another pitchstone-like rock 
from above Airthrey Castle in the Ochil Hills, near Bridge of Allan. It 
sometimes presents a nodular or coarsely perlitic character, weathering- out 
in nut-like balls, like the rock of Buckham’s Wall Burn in the Cheviot 
Hills.® Much more frequent is a weU-developed amygdaloidal structure, 
which indeed may be said to be tlie most obvious characteristic of these 
rocks as a whole. Tlie steam-vesicles, now filled with agate, (piartz, calcite 
or zeolite, vary in size from mere granules up to large irregular cavities a 
foot or more in diameter. MTiere the kernels are coated with pale-green earth 
and lie in a dark brown matrix, they give rise to some of the most beautiful 
varieties of rock in any volcanic series in this country, as may be seen on 
' An instance of this rock oceur.s in Kincardineshire, from wliicli the laro-e flat twin.s of 
lahradorite liave been analyzed hy Dr. Heddle {Trans. Roy. Soc. Min vof .\-xviii (187'n 
p. 257). ■ ■ 
^ C. T. Clough, “ The Cheviot Hills,” Mem, Geul. Survey (1888), p 12 ' 
» Ibid. p. 11. 
