OF THE EARTH'S SURFACE. 205 



inch ; but the killas has resisted that action completely, and its 

 fragments stand above the level of the corroded granite, pre- 

 senting to view the diluvian dressing unchanged. A curious 

 consequence has resulted from this circumstance : A fine white 

 clay, produced by the decomposition, was mistaken some years 

 ago for marl, and in order to obtain it, a tunnel was cut 

 through the rock, over which the water in its exit to the lake 

 had flowed. The level of the water having in this manner been 

 lowered several feet, a portion of the rock of the island is 

 brought into view, which had previously lain under water, and 

 which, in consequence of the exclusion of air, had not been de- 

 composed, nor had suffered any change, since it underwent the 

 diluvian dressing. The consequence is, that in the upper part 

 of the island, we see the granite corroded, and the killas en- 

 tire, whereas, near the water's edge, the surface of the rock 

 presents its diluvian dressing throughout ; and the two sub- 

 stances are only distinguishable by their colour. We have 

 thus an interesting display of the influence of time ; and the 

 circumstance suggests to me a mode, by which the antiquity 

 of these events may perhaps, in a certain degree, be ascertain- 

 ed, and of which some future observer may avail himself. 

 Had the tunnel been cut many centuries ago, at a known pe- 

 riod, and had the granite exposed by it to decomposition, un- 

 dergone a measurable corrosion, we should have been furnish- 

 ed, by instituting a comparison between the two corrosions, 

 with a chronometer, or a rational basis for calculating the anti- 

 quity of the diluvian event. The same view may be taken of 

 those blocks of granite in the valley of Monti, upon Saleve, 

 mentioned by Saussure, and alluded to in a former part of this 

 paper. 



In the neighbourhood of Loch Doon, a set of very large, 

 loose, and rounded blocks of stone occur, which resemble those 



to 



