XXVII. Detached Microliths from the Pitchstone Sill at Corriegills, Arran

Abstract

The Great Pitchstone Sill at Corriegills has often been examined and described. Bryce in his “Geology of Arran” (3rd ed. 1865, p. 70), says, “There is no remarkable change [produced by the pitchstone] on the sandstone—a slight induration merely—but the lower portion of the pitchstone is changed into a blue-coloured, porous, slag-like matter, like a pumiceous lava; this, however, is probably the mere result of decomposition.” That the pitchstone has been decomposed by percolating water finding its way between it and the sandstone—and which still oozes from that quarter—I think there can be no doubt, but I can see no resemblance in the decomposed part to the so-called “slag-like matter.” The following is my measurement of the bottom part of the pitchstone sill here, at a part of the cliff where it is easily accessible, its total thickness, according to Bryce, being 13 feet 5½ inches:— 1. Red Sandstone. 2. Pitchstone, laminar and columnar. 3. Pitchstone decomposed, but laminar, 5 inches. 4. Pitchstone decomposed, laminae not visible, joints still seen, 7 inches. 5. Pitchstone decomposed into yellow clay, neither joints nor laminae, 5 inches. 6. Red Sandstone. No. 3 can be dug out in thin layers which contain many irregular cavities caused by decomposition. When dry it is greenish in colour, and many white microliths can be seen in it with a pocket lens. Its surfaces are covered with minute circular patches which, where they run together, form figures like those seen on the polished surfaces of some jaspers. The intermediate spaces This 250-word extract was created in the absence of an abstract