File:The Quarterly journal of the Geological Society of London (14076960164).jpg

592
C. RICKETTS ON THE ERKATICS IN THE
thus surcharged -with mud may account for the entire absence of
marine life ; fractured, rarely perfect, shells occur sparsely scattered
in the clay, but neyer under such circumstances that it could be
imagined they had lived where found.
The pebbles and boulders imbedded in the clay, and from which
the formation derives its name, consist of fragments of hard rock
from the size of minute grains to two or three feet or more in
diameter, such as may have been derived from lands encompassing
the Bay of Liverpool and the adjoining portion of the Irish Sea —
from Cumberland, the south-west of Scotland, the north and east of
Ireland, and K'orth^^ ales. Their surfaces are very generally flattened,
smoothed, and polished, and a large proportion are covered with
striae, grooves, and scratches, universally acknowledged to have been
caused by abrasion beneath glaciers. If it is conceded that they
have dropped into the clay from floating ice, their number is such
as would indicate that the whole bay was sufiiciently packed with
bergs and floes to prevent altogether the formation of waves *, and
therefore, in the absence of other currents, no evidences of strati-
fication are afforded.
Amongst the erratics in the Boulder-clay may be included masses
of unconsolidated sands and gravels, often alluded to by local
geologists as " pockets of sand," &c. The materials resemble accumu-
lations already referred to as situated in the bottoms of valleys as a
bed upon which the Boulder-clay reposes ; their general shape is
comparable to that of the section representing masses containing a
remarkable collection of dark green blocks of disintegrated traps,
unmixed with other boulders, exposed in 1878 during the con-
struction of the Bootle Docks (fig. 1). These were imbedded in a
Pig. 1. — Section in Bootle Docks, Liverpool.
(Length aboat 28 feet.)
a. Trias. h. Gravel. c. Boulder-clay.
d. Masses containing disintegrated Trap.
light green sandy matrix, and formed accumulations which were
very conspicuous, the colour being in marked contrast to that of the
Boulder-clay ; their disintegration must have been due to the same

• " However great the agitation of the sea may be in the open ocean, and
  

though it may dash its waves with wild fury on the edge of the ice, within the
ice-girdle it is undisturbed" ('New Lands within the Arctic Circle,' by Lieut.
Julius Payer : chap. i. § 23).
In January 1881 the Mersey was covered with floating ice, or rather snow,
for nearly its whole width ; on the waves reaching the ice they terminated in a