Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/846

734 734 D. MACKINTOSH ON SOME NEW SECTIONS sand-and-gravcl occupj'ing deep hollows in, and resting on the rock- surface at the same level with the lower clay ; but these appear- ances (which are of frequent occurrence elsewhere) afforded no proof that both had been contemporaneously deposited. Though the upper clay in those sections contains fewer small stones than the lower, it is charged with a greater number of large boulders, con- trary to the general rule ; but it ought to be remembered that large boulders generally occur in groups instead of being equally scat- tered. Besides " greenstone," Eskdale granite is frequent among the boulders of the upper clay, though Criffell granite, so far as I could see, is limited to the lower clay or the gravels derived from it. A difference in the granites of the two clays is likewise seen at Dawpool and elsewhere, though I should not like to affirm that Criffell granite is everywhere entirely absent from the upper clay. Sections around Birkenhead, showing the Relations between the Boidder-clays and the underlying Rock-surfaces. — On both sides of the Borough lioad, between the new Mission-house and the water- works, a number of brick-pit and other sections have lately been exposed. Behind the Mission-house an excavation (April 1877) shows, under a deposit of upper Boulder-clay, a Triassic rock-surface planed down unconformably to structure, in a manner suggesting the sudden and forcible grounding of a mass of floating ice. A few yards to the south-west, a continuation of the planed surface is covered with grooves (afterwards to be described). Further on, at a somewhat higher level, a similarly planed-down rock-surface may be seen. Higher up on the other side of the road a number of excavations show : — (1) the lower Boulder-clay or its hard loamy representative graduating downwards into an unwashed Boulder- gravel, consisting of rounded erratic stones and angular sandstone fragments worked up from the underlying rock ; (2) the same clay reposing in one place on an uneven, in another on a straight surface of rock or rock-sand in situ ; (3) the upper Boulder-clay, never graduating into rock or rock-sand downwards, but separated from them sometimes by an uneven, but generally by a straight line. In digging the foundations for the head master's new house, Birkenhead School, the upper Boulder-clay in several places was seen to rest on a cleanly shaved-off surface of coloured Triassic sand in situ. Derivation of the Component Materials of the Boulder-clays. — In the neighbourhood of the estuaries of the Dee and Mersey, and, I have no doubt, in most places where either or both of the Boulder- clays exist, the following description will, I believe, be found more or less applicable. A great part of the two formations consists of broken-up or ground-down Triassic sandstone (in some districts evi- dently of Permian or Carboniferous sandstone). In specimens from the sections above described, the residue, after washing, consists of coarso sand, which, to a great extent, is composed of rounded and subangular quartz grains, and small subangular stones from Bunt el- and other Triassic sandstones. The erratic stones, which are seldom very small, were apparently brought by a cause distinct from that