1854.] BRICKENDEN — GLACIAL TRACES, DUMBARTON, 



29 



that the occurrence of glacial traces beneath an overhanging rock 

 bordering a fissure at Dumbarton Castle is, amongst others, worthy 

 of being recorded. 



The hard whinstone, which here rises up abruptly from the allu- 

 vial shores of the River Clyde, exhibits in a very interesting and 

 beautiful manner the crystalline and prismatic structure peculiar to 

 such trappean effusions, but the lines of fracture or divisional planes 

 of the Rock of Dumbarton are observed to inclhie generally at an 

 angle of about 70°, giving to the Rock on one of its sides a rather 

 precipitous and overhanging appearance. The effect of denudation 

 or abrasion has therefore been to wear or score it in certain parts into 

 fissures, the sides of which have a parallel inclination ; and across 



Fig. 3. — Plan of the Rock of Dumbarton Castle. 



a. Fissure. 



the centre of the Dumbarton Rock there is one of considerable depth, 

 di\'iding the hill into two parts or summits, as shown in fig. 2. 



It is on the sides of this fissure, which now forms a passage, and 

 the only mode of approach to the buildings on the Rock, that striae 

 and abrasions are observed, which constitute the subject of this notice. 



The fissure intersects the Rock from south to north, pointing 

 directly up the valley of the Leven, through which in the distance 

 appears the lofty summit of Ben Lomond. The wall of rock on the 

 left hand or western side of the fissure about midway up the ascent. 



