166 
THE GEOLOGIST. 
observed on the banks of Loch Doon, in Ayrshire, as the remarkable 
striated trap-rocks cannot be so easily removed. The openings or 
striae are generally narrow, long, and deep ; sometimes prismatic. 
They appear to resemble in sharpness the scratchings considered, by 
Dr. John Tvndall, in his published ' Description of the Glaciers of 
Fig. 6. — Striated basalt, Loch Doon (as seen from the Lake). 
the Alps,' to be caused, on the Eiffelhorn, by the Gorner glacier of 
former ages. The boulders on the shores of Loch Doon are fre- 
quently very large, and mostly basalt ; and so numerous, as to con- 
stitute entirely the west side of the loch, the waters of which appear 
to rest on a bed of boulders, _ 
four of which are so large 
as to appear above the ^ - 
surface, and now remain ^ " 
islands. (See sketch.) 
Koue of the numerous Tig. 7.— Island-boulders, Locb Doon. 
boulders examined by me possess the strong and varied polarity 
which Dr. Tyndall found to exist in some cast down from the 
summit of the Eiffelhorn. It is not maintained that the many 
trap-boulders scattered through Scotland were never magnetic ; and 
it is only affirmed that no fragment has yet been discovered that 
was observed to act forcibly upon a magnetic needle. It may be here 
remarked, that many of the "jutting prominences" of Dumbarton 
basalt rock, on which the fortress is situated, like those of the Eiffel- 
horn and Gorner Grat, described by Dr. Tyndall, possess the mag- 
netic action caused by the magnetic oxide of iron. Has this 
magnetism been developed by glacial action ? — is a question which 
cannot be here answered, although it is one worthy of being con- 
sidered by geologists. The top and base of Dumbarton Eock 1 have 
examined with a small compass. ]\ear the summit of the portion of 
this rock called the beacon-tower, the polarity changed, the pole 
alfected being the south, at the distance of 34 inches. In no instance 
did I observe the compass to be affected in the east portion of the 
rock, either at the top or base ; the reverse was the case with the 
west portion, both at the top and base, the immense fragments, un- 
doubtedly portions of the rock, removed probably, by lightning, on the 
terrace at the foot of this part of the rock, being no exception to the 
" strong and varied polarity." 
Glacier pressure is considered by Dr. Tyndall to cause the crust 
