Correspondence — Mr. D. Mackintosh. 191 



cannot help believing that land-ice or floating-ice would have 

 furnished a more uniformly-directed and horizontally-operating cause 

 of the curving back of slaty larainge over a large area on the flat 

 summit of Brendon Hill than the agency principally advocated by 

 Mr. Ussher, namely, " oft-repeated internal movements," producing 

 curves, flexures, and contortions, and revealed at the surface by the 

 planing action of denuding agents. The chances against the irre- 

 gularly-degrading action of subaerial denuding agents having stopped 

 short over a large area along the horizontal plane w^here the summit 

 of the curved-back slaty laminae occurs, must have been exceedingly 

 small, while the presence of blocks of quartz imbedded in the more 

 shattei'ed parts of the curved-back lamina (blocks which must have 

 been carried, or pushed forward, on perfectly level ground, to con- 

 siderable distances from their native veins) cannot be reconciled 

 with the internal movement theory. Mr. Ussher regards the "Head" 

 as a subaerial accumulation, but Sir E. I. Murchison long ago, and 

 Mr. Belt and Prof. Prestwich lately, have proved that it must have 

 been deposited under the sea or an immense ice- water lake. It may 

 yet turn out to be equivalent to the Upper Boulder-clay of the 

 North-west of England. Neither can I agree with Mr. Ussher in 

 supposing that the south-western counties, any more than the north- 

 western, underwent a " great surface-waste and contour-moulding in 

 Pleistocene times." In the north-west no fact forces itself more on 

 the attention than the Preglacial origin of all the leading varieties 

 of surface-configuration, especially the valleys. This, I believe, is 

 admitted by all geologists who have studied the subject. 



D. Mackintosh. 

 P.S. — Since the above was written, I have noticed that Mr. Ussher 

 regards the "intrusion of roots acting as wedges" as the "most 

 common cause of strictly superficial curvature." In all the instances 

 described in my paper in the Quart. Journ, Geol. Soc, for November, 

 1867 (excepting the one at Gupworthy), the curving back of the 

 slaty laminse is confined to a space only a few feet in depth. A very 

 little reflection must show that the intrusive action of roots could 

 never have persevered in one direction in bending back the inclined 

 lamince against their nap. With regard to the comparative absence 

 of curved-back laminse on the northern slope of Brendon Hill, if 

 the direction and high angle of the cleavage dip be there the same 

 as on the summit of the hill, Mr. Ussher is not right in implying 

 that ice moving up to the northern slope would encounter more 

 resistance from the nap of the laminse than on the summit, as a 

 simple diagram will show. D. M. 



D\d:iSC!E3LXi-A.3sr:EOTJS. 



Society of Arts Blowpipe Prize. — The Council of the Society 

 of Arts has awarded to two Cornishmen, Messrs. Letcher of St. Day 

 and Camborne, the Silver Medal of the Society, and a Prize of £10, 

 for the best set of Blowpipe Apparatus which could be sold retail 

 for One Guinea. 



