30 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 15, 



inclines over the passage at an angle of about 70°, which is the pre- 

 vailing inclination of the columnar divisions of the whinstone, aijd 

 chiefly here on its very hard and durable surface are seen engraved, 

 very distinctly, furrows and striae ; indicating the full force of the pro- 

 pelling agent and its instrumentality, whatever that may have been, 

 in the most contracted and cavernous part of the fissure. On the 

 opposite or eastern side it appears that the rock has been artificially 

 removed at some time for the purpose of widening the passage, which 

 even now does not exceed 1 feet in width ; but on the upper part 

 very well-defined scratches are seen to maintain a direction corre- 

 sponding to those on the western side. The striae are not horizontal, 

 but run in lines nearly conformable to the declivity of the passage. 

 As, however, these striae are seen on an impending surface of rock, 

 nothing can be learnt from them as to the precise point by compass, 

 from which the abrading agent may have proceeded ; and all that 

 we can now perceive of its action is that which was confined within 

 the narrow fissure, into the northern entrance of which it had intruded 

 itself. That part of the Rock on which these abrasions are seen 

 stands above the Clyde at an elevation of about 150 feet, and above 

 this the Rock rises precipitously about an equal number of feet to its 

 highest point. 



In the accompanying sketches (figs. 1, 2, 3) are shown, by transverse 

 and longitudinal sections, the form and dimensions of the fissure, 

 both as it now is, and as it probably appeared previous to the period 

 of human history. 



The Rock of Dumbarton rears its bold front immediately in the 

 central line of the valley of the Leven, through which the small 

 river of the same name flows in its meandering and alluvial channel 

 from Loch Lomond to the Clyde ; and it must at all times have pre- 

 sented a direct barrier or obstruction to whatever has descended in 

 that channel from its highland source. 



At the foot of the Rock on its northern side a strong tenacious 

 clay, evidently belonging to the boulder-formation, descends beneath 

 the level of the tide ; this is in a great measure obscured by a talus ; 

 and at a short distance from the Rock it is covered by the alluvial 

 deposits of subsequent periods. But in this boulder clay we recog- 

 nize the remains of that extensive glacial detritus which at the con- 

 clusion of the epoch of its accumulation very probably existed to a 

 great depth in this part of the Leven Valley, and more particularly 

 where its transport would be arrested by the abrupt escarpment of 

 the Rock. 



