[328] There was a trace of manganous oxide in this specimen.
(b) The Andesites, or, as they were formerly called, Porphyrites, which constitute by far the largest proportion of the lavas, have a characteristic but limited range of lithological varieties. The prevailing type presents a close-grained, rather dull texture, and a colour varying from pinkish grey, through many shades of green and brown, to purplish red, which last is, on the whole, the predominant hue. Minute lath-shaped felspars may frequently be detected with the naked eye on fresh surfaces, while scattered crystals, which are generally hæmatitic pseudomorphs after some pyroxene, occasionally after hornblende 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 brown or black mica. Where large felspar-crystals occur in a compact green matrix, the rock assumes a resemblance to the verde antique of the ancients.[329] One of the Cheviot andesites lying at the bottom of the series is distinguished by its large and abundant plates of black mica.[330]
[329] An instance of this rock occurs in Kincardineshire, from which the large flat twins of labradorite have been analyzed by Dr. Heddle (Trans. Roy. Soc. Edin. vol. xxviii. (1879), p. 257).
[330] C. T. Clough, "The Cheviot Hills," Mem. Geol. Survey (1888), p. 12.
The texture of the andesites occasionally becomes faintly resinous, where a considerable proportion of glass still remains undevitrified, as in the well-known varieties from the Cheviot 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.[331] Much more frequent is a well-developed amygdaloidal structure, which indeed may be said to be the most obvious characteristic of these rocks as a whole. The steam-vesicles, now filled with agate, quartz, calcite or zeolite, vary in size from mere granules up to large irregular cavities a foot or more in diameter. Where 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 the Ayrshire coast at Culzean and Turnberry. Some rocks contain the vesicles only as rare individuals, others have them so crowded together as to form the greater part of the cubic contents of the mass. When the infiltration-products have weathered out, some of the amygdaloids present a striking resemblance to recent slaggy brown lavas; lumps of them must have been originally light enough to float in water.
[331] Ibid. p. 11.
My colleague in the Geological Survey, Mr. J. S. Grant Wilson, some years ago made for me a large series of determinations of the specific gravity of the volcanic rocks of the Lower Old Red Sandstone of Scotland. He found that the andesites collected from various districts to illustrate the more typical varieties of rock averaged about 2·66. He also made a series of chemical analyses of a number of the same rocks from the Cheviot Hills, where they are well preserved. The results are shown in the following table:—
| SiO2 | Al2O3 | Fe2O3 | FeO | MnO | CaO | MgO | K2O | Na2O | H2O | H2SO4 | Loss. | |
| Scawd Law | 59·29 | 16·30 | 1·77 | 3·70 | ·41 | 4·81 | 3·15 | 4·19 | 3·44 | 3·84 | ... | ... |
| Rennieston | 62·81 | 16·40 | ·55 | 3·27 | ·81 | 4·46 | 1·64 | 3·60 | 3·02 | 4·04 | ... | ... |
| Cunrieston | 63·38 | 15·77 | ·73 | 2·65 | ·08 | 4·44 | 1·88 | 1·88 | 4·54 | 4·69 | ... | ... |
| Duncan's Dubs | 59·44 | 16·15 | 1·05 | 2·83 | ·37 | 6·70 | 2·46 | 3·18 | 3·70 | 3·35 | ... | ... |
| Whitton Hill | 60·70 | 17·98 | ·66 | 2·58 | ·20 | 7·07 | 2·20 | 3·57 | 2·95 | 3·45 | ... | ... |
| Cuddies' Tops | 60·58 | 12·25 | 1·01 | 4·13 | ·15 | 4·40 | 2·86 | 2·19 | 3·61 | ... | ·55 | 2·15 |
| Cocklawfoot | 62·29 | 17·03 | ·93 | 2·44 | ·21 | 3·92 | 2·71 | 1·14 | 3·20 | ·29[332] | ·37 | 4·81 |
| Morebattle | 59·82 | 16·96 | ·20 | 6·57 | ·15 | 4·73 | 2·84 | 2·63 | 3·04 | ... | trace | 1·98 |
[332] This is CO2.
The microscopic structure of the andesites of the Lower Old Red Sandstone has been partially investigated, especially those of the Cheviot Hills, by Mr. Teall[333] and by Dr. Petersen,[334] who both give chemical analyses of the rocks. Much, however, still remains to be done before our knowledge of this branch of British petrography can be regarded as adequate. The groundmass in some of the rocks consists mainly of a brown glass with a streaky structure (as in the well-known variety of Kirk Yetholm, and in the rock, still more like pitchstone, from near Airthrey Castle in the Ochil chain); more usually it has been devitrified more or less completely by the appearance of felspathic microlites, until it presents a confused felspar aggregate. The porphyritic felspars are often large, generally striped, but sometimes including crystals that show no striping. They are frequently found to be full of inclusions of the base, and these sometimes consist of glass. The ferro-magnesian constituents are usually rather decomposed, being now represented by chloritic pseudomorphs; but augite, and perhaps still more frequently enstatite, may be recognized, or its presence may be inferred among them. The beautiful resinous or pitchstone-like rock from near Airthrey Castle has been found by Mr. Watts to be a glassy hypersthene-augite-andesite, since among its phenocrysts of plagioclase, augite and hypersthene 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.[335]