1854.J BRICKENDEN GLACIAL TRACES, DUMBARTON. 27 



, EXPLANATION OF PLATE L 



Map and Sections to illustrate Mr. D. Sharpe's Paper on the 

 neighbourhood of Mont Blanc. 



The object of the Map and Sections being to show the direction 

 of the planes of foUation and cleavage, only the principal geological 

 features are represented, and four colours employed. 



The Yellow represents all the unstratified foliated rocks ; including 

 Gneiss, Protogine, Mica Schist, and Talc Schist. 



The Purple represents the stratified Slates ; including all the stra- 

 tified deposits intersected by slaty cleavage. 



The Blue represents the beds of Jurassic Limestone, which are free 

 from cleavage. 



The Crimson shows the intrusive igneous rocks of more modern date 

 than the cleavage. 



The Black lines on the Map indicate the strike or direction of the 

 planes of foliation and cleavage ; where they cross the rocks 

 coloured yellow, they represent the foliation ; where they cross 

 the purple colour, they show the cleavage. These lines are 

 double where the foliation or cleavage is vertical, single along 

 the central axis of an arch or anticlinal of foliation or cleavage. 

 The angles of inclination of the planes along the axes and between 

 the axes and the vertical planes can be learned from the sections, 

 as the scale of the Map is too small to admit of the intervening 

 planes being laid down. The lines are dotted where they have 

 not been actually observed, but their direction has been 

 inferred either by continuing them from the localities observed, 

 or by drawing them parallel to the strike of the intervening 

 variously inclined planes which have been observed, but which 

 have not been laid down on the Map from want of space. 



The ground-plan of the Map is copied from the one-sheet map of 

 the kingdom of Sardinia, reduced by Andriveau-Goujon of Paris from 

 the larger official map. 



The Sections Nos. 1 to 5 are drawn on parallel lines across the 

 chain of Mont Blanc, No. 1 lying most to the North, No. 5 most to 

 the South, and their directions are indicated by lines on the Map, 

 marked § 1 to § 5, They are all nearly on the same scale, with the 

 height somewhat exaggerated : the dotted lines represent the planes 

 of foliation and cleavage. Each line of vertical planes of foliation and 

 cleavage is marked by the same letter, and each anticlinal of those 

 planes by the same number; thus all the points marked alike are on 

 the same line of strike, and the lines on the Map are similarly marked. 



2. On the Occurrence of Glacial Traces on the Rock of Dum- 

 barton. By Capt. L. Brickenden, F.G.S., of Dumbarton 

 Castle. 



As the very peculiar and remarkable positions in which glacial striae 

 and abrasions are sometimes observed on the surface of rocks in 



