382 SECTIONAL TRANSACTIONS.—C. 
xenoliths, drusy facies and columnar joints in the ailsyte ; to the distribution 
and petrography of the dolerite and basalt sheets, and to a raised beach at 
fifteen feet above present mean high-water mark. 
The Craig has the form of a blunt sugar-loaf, rising to a height of 1,114 ft. 
above the sea at a point nine and a half miles due west of Girvan, Ayrshire. 
In plan it is sub-rectangular, with its greatest length (over three-quarters of 
a mile) north-south, and its greatest breadth (over half a mile) east-west. 
On the south and west are vertical, columnar cliffs, while on the north- 
east and east are more stable joint-controlled slopes ; from the east coast 
a cusp-shaped spit of shingle projects for nearly three hundred yards, and 
storm beaches encircle the isle. , 
The ailsyte occurs in coarse and fine varieties, known, locally, as the 
common rock and the red and blue hones respectively. The coarse rock, which 
includes drusy types, has been used in the past for road metal and monu- 
ments ; the fine varieties are still sold for curling stones. A few small but 
well-preserved sedimentary xenoliths have been found by the author in 
both varieties. 
The basic dykes, which number about twenty, are sub-vertical sheets of 
coarse and medium dolerite or basalt, as much as thirty-nine feet thick, 
with a dominant north-south trend but a local zigzag course round groups 
of ailsyte columns. Weathering in these rocks has given rise to parallel 
‘steps’ and clais (Gaelic for a ‘ trench’) structures on the slopes of the 
Craig, and to caves of flask-like profile at present sea-level as well as at the 
height of the fifteen-foot raised beach. The latter is marked by pebble 
deposits bearing erratics, and an impersistent coastal notch. 
Dr. S. W. Wootpriwce.—The glaciation of the London basin and the' 
evolution of the Tertiary escarpment in Hertfordshire (11.0). 
This communication directs attention to certain problems connected with 
the belt of drift deposits containing Triassic debris and extending from 
Goring Gap to Hertford, along the northern margin of the London Basin. 
Dr. R. L. Sherlock contends that the pre-Glacial Thames followed this 
course. Other workers have examined parts of the belt and reached partial 
conclusions. A thorough re-examination of the evidence has been under- 
taken to see whether Dr. Sherlock’s hypothesis can be substantiated or 
amplified, and what alternative possibilities should be considered. 
The morphological features of the region considered alone point to con- 
clusions difficult to reconcile with those derived from the drift deposits. 
In western Herts, two wind-gaps in the Tertiary escarpment appear to 
indicate drainage across the line of the Vale of St. Albans at a relatively late 
date in the evolution of the region. ‘The eastern end of the escarpment 
is unbreached by wind gaps and any cross drainage must have antedated 
the present cycle of erosion. This line of argument, however, leads to a 
complete dilemma when the drift deposits of the Vale are considered. At 
least two sets of glacial or fluvioglacial deposits occur separated by 100 ft. or 
more in level. The lower series represents ice advancing from the east after 
the abandonment of the wind-gaps. The upper series, largely fluvio- 
glacial, is apparently of westerly provenance, but the relation of the drift to 
the wind gaps is very difficult to interpret—and the reading of the physio- 
graphy of the whole region must turn on this point. 
The problems at issue were discussed in the field during the Centenary 
Meeting in London, and this paper offers a further contribution to an 
unsolved problem. ' 
REPORTS OF RESEARCH COMMITTEES (11.30). 
