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 



