[337] Notes by Dr. Hatch.
At the other end of the series come the felsites, quartz-porphyries, mica-porphyrites, minettes, vogesites, "hornstones" and "claystones" (or decayed felsites), which have a close-grained texture, often with porphyritic felspars, quartz or black mica, generally a whitish, pale buff, orange, pink or purplish-grey colour, and a specific gravity of about 2·55.[338]
[338] The intrusive "porphyry" of Lintrathen in Forfarshire (which may be younger than the Old Red Sandstone) is a bright red rock with porphyritic felspar, quartz, white mica and a very singular black mica (Mr. Teall's British Petrography, p. 286).
Though I class these rocks as intrusive, I am not prepared to assert 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 cases the evidence either way is doubtful, but as the great majority of the acid rocks can be 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 of felsitic debris was ejected to form the tuffs, more of this material may have flowed out in streams of lava than is at present recognized.
The following table shows the chemical composition of some acid sills and dykes from the Lower Old Red Sandstone, as determined in the laboratory of Prof. E. Frankland: [339]—
[339] Two analyses of rhyolites from Shetland by Mr. Tatlock will be found in Trans. Roy. Soc. Edin. vol. xxxii. (1887), p. 387. Their silica percentage is 72·32 and 73·70. An analysis of a quartz-felsite from the Cheviot Hills by Mr. T. Waller is given in the Geological Survey Memoir on the Cheviot Hills, p. 25. The proportion of silica in this rock is 67·9.
| SiO2 | Al2O3 | Fe2O3 | MnO | CaO | MgO | K2O | Na2O | P2O3 | H2O | |
| "Hornstone." Torgeith Knowe, Pentlands | 73·91 | 14·41 | ·76 | ·07 | 1·21 | 4·90 | 3·36 | 1·57 | ... | ·90 |
| "Hornstone." Braid Hills[340] | 64·73 | 17·01 | 2·35 | ·24 | 4·19 | ·66 | 3·27 | 3·75 | ·26 | 2·78 |
| Tinto, Lanarkshire: | ||||||||||
| Soluble in hydro- chloric acid | ·04 | 1·01 | 1·24 | ... | ·92 | ·52 | ... | ... | ·16 | ... |
| Insoluble in ditto. | 70·28 | 12·54 | ·43 | ... | ·91 | ... | 3·92 | 5·84 | ... | 1·99 |
[340] This specimen also yielded 0·13 of ferrous oxide, and 2·42 of carbon dioxide.
The rock of Tinto, which may be considered typical of the prevailing acid intrusive rocks of the series, presents several slightly different varieties. Dr. Hatch, as the result of his examination of a number of microscopic slides prepared from specimens taken by 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 brown, finely microlitic groundmass of felspar powdered over with calcite; 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 through the other rocks[341] (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 pseudomorphs after that mineral, pass into vogesites.
[341] This rock differs considerably from the other intrusive masses in its neighbourhood. Dr. Hatch found it to be composed chiefly of lath-shaped striped felspar, with some granular augite, magnetite and interstitial quartz.