278 
VOLCANOES OF THE LOWER OL.D RED SANDSTONE 
BOOK V 
shown to be intrusive in their behaviour, I have preferred to keep them all 
in the same category. I am prepared to find, however, that, as so vast an 
amount ot felsitic debris was ejected to form the tuffs, more of this material 
may have flowed out in streams ot lava than is at present recognized. 
The following table shows the chemical composition of some acid sills 
and dykes from the Lower Old Eed Sandstone, as determined in the 
laboratory of Prof. E. Frankland:’ — 
SiOo 
AJ2O3 
FeaOa 
iMuO 
CaO 
MgO 
KoO NauO 
P 2 O 5 
H 3 O 
“ Horustone.” 
Torgeith Knowe, 
Peiitlands . 
73-91 
14-41 
-76 
-07 
1-21 
4-90 
3 -.36 
1-57 
-90 
“ Horii.stone.” 
Braid Hills*. 
64-73 
17-01 
2-35 
-24 
4-19 
-66 
3-27 : 
3-75 
-26 
2-78 
Tinto, Lanarkshire ; 
Soluble in hydro- 
chloric acid 
Insoluble in ditto . 
-04 
70-28 
1-01 
12-64 
1-24 
-43 
-92 
-91 
-52 
... 
3-92 
5-84 
-16 
i-99 
* This specimen also yieldeil 0-13 of ferrous oxide, and 2-42 of carbon dioxide. 
The rock of Tinto, which may he considered typical of the prevailing 
acid intrusive rocks of the series, presents several slightly different 
varieties. J3r. Hatch, as the result of his examination of a number of 
microscopic slides prepared from specimens taken liy me from various parts 
of the hill, found some to be minettes, showing small isolated crystals of 
orthoclase and rare flakes of biotite, sometimes granules of quartz, imbedded 
in a lirown, finely microlitic groundmass of felspar powdered over with 
caleite , while other specimens had a granular instead of a microlitic 
groundmass, and contained a considerable amount of quartz in addition to 
the constituents just mentioned. A conspicuous knob on the south side of 
Tinto, called the Pap Craig, is a mass of augite-diorite, which has risen 
tlirough the oEier rocks'’ (see Fig. 93). The sills in the same region show 
still further differences. Some are true “ felspar-porphyries,” and “ quartz- 
porphyries ” varying in the relative abundance and size of their porphyritic 
orthoclase and quartz, while others, by the introduction of hornblende or 
pseudoniorphs after that mineral, pass into vogesites. 
Basic sills and bosses are chiefly developed among the Ochil and Sidlaw 
Hills. They may generally be classed as diabases. But sometimes their 
pyroxenic constituent is partly hyperstheiie, as in a coarsely crystalline boss 
about a mile south of Dunning, which has been determined by Mr. Watts 
) Two analyses of rhyolites from Shetland by Mr. Tatloek will be found in Trans. Roy. Soc. 
Dhii. vol. .\xxn. (188/), p. 387. Their silicafpercentage is 72-32 and 73-70. An analysis of a 
-imirte-lelsite trom the Cheviot Hilts by Mr. T. Waller i.s given in the Geological Survey 
Memoir mi the Cheviot Hills, p. 25. Tlie proportion of silica in this rock is 67-9. 
- This rock (litters considerably from the otlier inti-uaive masses in its neighbourhood. I)r. 
Hatch found it to bo composed chielly of lath-shai.ed .striped felspar, with some granular augite 
magnetite and interstitial quartz. ' 
