ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxxi 



planes of false stratification ; interstratified with these sandstones, and 

 usually towards their base, are sundry beds of coarse breccia, consist- 

 ing of angular fragments of the identical rock against which the de- 

 posit rests, — that is, of slate where that is slate, and of traj) at Drum- 

 lanrig where the breccia abuts against trap. These beds are every- 

 where at a considerable angle, varying from 12° to as much as 30°, 

 and in every instance they abut against a steep escarpment of rock^ 

 which, if stratified, dips in an opposite direction to the sandstones. 



It had been stated by previous observers that these various depo- 

 sits of sandstone all dipped in the same direction ; which circumstance 

 might have been explained by supposing the whole district to have 

 been elevated from the east, giving a steep westerly dip to all the 

 previously horizontal deposits of sandstone which lay in the bottoms 

 of the valleys. But this conjecture is effectually disposed of by the 

 more accurate observations of Professor Harkness, that the sandstones 

 dip in different localities N.N.W., W.S.W., S., S.E., S.S.E., and N.; 

 and he therefore suggests that a local depression or sinking of 

 portions of the district in which the sandstones occur may have 

 brought them all to their present anomalous position. 



I do not recollect any other instance of a set of deposits, which 

 only occupies one side of the valley in which it occurs, abutting 

 at a high angle against the escarped edges of the rocks which, 

 dipping away from the valley, bound it on that side ; and is altogether 

 wanting on the smoother side of the valley, which is formed of beds 

 dipping towards the middle of the valley : and every explanation 

 must be defective which does not account for this peculiarity. 



The facts so clearly stated by the author have suggested to me the 

 idea that these deposits are portions of a talus of broken fragments 

 which fell from the steep faces of the overhanging mountains into 

 basins of water at their feet ; the materials of such a talus being neces- 

 sarily composed of the overhanging rock ; the larger fragments are 

 angular ; the smaller, having been disintegrated by rains and other 

 atmospheric agencies, have attained the condition of sand or mud : 

 the high inclinations of the beds of slaty rock in these localities have 

 formed an important element in the cases before us, as their over- 

 hanging edges would supply such angular fragments in large quan- 

 tities ; but on the opposite side of the valley, where the dip of the 

 slates nearly coincides with the slope of the hill-sides, no such break- 

 ing off of fragments of rock would take place, and little or no talus 

 would be formed, as the only materials carried down would be the 

 finer matter washed down by rain and carried to the bottom of the 

 valley. 



The fallen materials constituting a talus at the foot of a cliff or 

 steep escarpment arrange themselves in the open air at an angle of 

 about 35° ; when they fall into still water, the angle is less ; and, 

 if the water into which they fall has any motion, either from 

 tides or a current, the angle will be materially less, depending 

 on the velocity of motion of the water. The angles at which the 

 breccias are found, and every other condition of the case before 

 us, may thus be accounted for on the simplest principle, without 



VOL. XIII. / 



