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 Tyndall, 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 Biffelhorn, 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, r 

 four of which are so large 

 as to appear above the 

 surface, and now remain 

 islands. (See sketch.) 

 None of the numerous Fig- 7— Island-boulders, Loch 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 Biffelhorn. 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 Biffel- 

 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 Bock 1 have 

 examined with a small compass. JN T ear the summit of the portion of 

 this rock called the beacon-tower, the polarity changed, the pole 

 affected 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 

 Avest 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 



