i02 Frof. Hughes, Criticism of the Geological [Oct. 30, 



Blown sand also gives a delicate polish but as in the former 

 case, see Nos. 16 and 17, slight differences of texture in the rock 

 are apt to give rise to inequalities which are perpetuated in a 

 fretted surface. This eating away of the rock by the constantly 

 impinging grains of sand is seen on the obelisks of Egypt, on the 

 basalt of Burntisland No. 16, or on the siliceous pebbles sticking 

 in the exposed cliffs of St Davids No. 17. On softer material its 

 action can be more easily studied, as for instance on pieces of 

 broken bottle lying on the shore in the track of the prevalent 

 wind. It has been turned to account in the sand blast used for 

 etching glass. 



General McMahon^ has described how gullies amongst the 

 rocks bring heavy gusts of wind to a focus, as it were, as may be 

 seen at Dosi an isolated hill within the Arvali area, where deep 

 grooves, several feet in depth and in diameter, have been carved 

 out of the sides and faces of huge granitic blocks b}^ sand-laden 

 wind aided by the selective agency of natural decay. 



Some remarkable specimens, Nos. 18 and 19, were noticed in 

 1869 by Mr Hackworth and described by Mr W. T, Travers before 

 the Wellington Society- and afterwards by Mr J. D, Enys before 

 the Geological Society of London^ These owe their form to the 

 fact that they lay in the route of prevalent winds compelled by 

 the form of the ground to carry the sand in a constant direction 

 back and fore. The exposed part of the stone was thus chamfered 

 off from each side so as to leave a roof-like ridge projecting. 

 These stones have no strise but generally show some places fretted 

 away below the general level in a manner that proves that they 

 were not planed or ground down by contact with flat surfaces. I 

 have found a siliceous pebble on the Plateau Gravel above Biarritz 

 No. 20 which was beginning to assume a similar form, from having, 

 as it appeared, been bevelled off by the sand blast driven by the 

 Atlantic gales which so often sweep that exposed ground. 



V. Glacially Striated Rocks. 



So far we have been considering chiefly the various agents 

 which produce smooth or polished surfaces and endeavouring to 

 discriminate those due to ice action from those due to other 

 causes. We will now examine the striated surfaces so commonly 

 found on the surface of the rocks over which ice has passed or on 

 the boulders which have been transported by it, and in this case 

 again we shall find that the mere presence of scratches and grooves 

 is not of itself sufficient evidence of glacial action. 



^ Records Geol. Survey of India, Vol. xvii. Pt. 3, 1884, p. 101. 

 ^ Trails. Neiu Zealand Listitute, Vol. ii. p. 246. 

 3 Q.J.G.S., Vol. xxxiv., 1878, p. 86. 



