. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 698 CARBONIFEROUS VOLCANIC ROCKS. [Ch. XXXII. The other or older class of carboniferous traps are traced along the south margin of Stratheden, and constitute a ridge parallel with the Ochils, and extending from Stirling to near St. Andrew's. They con- sist almost exclusively of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are regularly interstratified with the sandstone, shale, and ironstone of the lower Coal-measures, and, on the East Lomo
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 698 CARBONIFEROUS VOLCANIC ROCKS. [Ch. XXXII. The other or older class of carboniferous traps are traced along the south margin of Stratheden, and constitute a ridge parallel with the Ochils, and extending from Stirling to near St. Andrew's. They con- sist almost exclusively of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are regularly interstratified with the sandstone, shale, and ironstone of the lower Coal-measures, and, on the East Lomond, with Mountain Limestone. I examined these trap rocks in 1838, in the cliffs south of St. An- drew's, where they consist in great part of stratified tuffs, which are curved, vertical, and contorted, like the associated coal-measures. In the tuff I found fragments of carboniferous shale and limestone, and intersecting veins of greenstone. At one spot, about two miles from St. Andrew's, the encroachment of the sea on the cliffs has isolated several masses of trap, one of which (fig. 731) is aptly called the " rock and spindle," * for it consists of a pinnacle rig. TS2. of tuff, which may be compared to a distaff, and near the base is a mass of columnar greenstone, in which the pillars radiate from a centre and appear at a distance like the spokes of a wheel. The largest diameter of this wheel is about twelve feet, and the polygonal terminations of the columns are seen round the circumference (or tire, as it were, of the ?onemSen°feSSse wlieel)> as in the accompanying figure. I conceive at 5, fig. T3i. , this mass to be the extremity of a string or vein of greenstone, which, penetrated the tuff.« The prisms point in every direction, because they were surrounded on all sides by cooling surfaces, to which they always arrange themselves at right angles, as before explained (p. 617). A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in .the no
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
