164 David Milne, Esq., on Polished and Striated Rocks. 



corresponds with that of the individual crevasses, whose 

 grouping subdivides the glacier as I have now described. The 

 crevasses may be nearly transverse to the glacier, whilst the 

 si/stems of crevasses form an angle of perhaps 30° with the 

 transverse line. The veined structure again cuts the crevasses 

 at right angles, so that these may be regarded as three 

 orders of discontinuity, or tearing surfaces, which occur in 

 systetns, that is, regularly repeated at nearly uniform intervals 

 over great portions of the glacier surface. I have this year 

 succeeded, for the first time, in laying down on a map, an 

 approximation to the various and complex systems of cre- 

 vasses which traverse the Mer de Glace, and I have found 

 a repetition of this phenomenon of a series of discontinuous 

 but parallel fissures ranged along a line or axis oblique to their 

 direction, to recur at several points where the strain is very 

 violent. Let it be remarked, too, that where the violence of 

 the pressure opens a system of such fissures to relieve it, the 

 bands, or system of surfaces of molecular discontinuity, dis- 

 appear, or are less well developed. I remain, my dear Sir, 



yours sincerely, 



James D. Forbes. 



12tu December 1846. 



On Polished and Striated Bocks, lately discovered on Arthur 

 Seat, and other places near Edinburgh. By DAVID MiLNE, 

 Esq., one of the Secretaries of the Royal Society of Edin- 

 burgh.* 



For the last thirty years, much interest has prevailed among geo- 

 logists, connected with those appearances on the surface of hard rocks, 

 which clearly indicate that they have been ground down or polished, 

 and sometimes furrowed or scratched, by a foreign agent. So long ago 

 as 1812,'l" the late Sir James Hall called attention to the dressings 

 (as he termed them) of the rocks on Corstorphine Hill, consisting of 

 smoothed surfaces, marked also by ruts and scratches, all more or 

 less parallel to each other. 



* Read before the Royal Society of Edinburgh, on 20th April 1846. 

 t Transactions of the Royal Society of Edinburgh, vol. vii. 



