O. Poulett Scrope — On " Blocky'' Rock Surfaces. 291 



conglomerates of Old Eed Sandstone and Carboniferous age in 

 Scotland, I fully concur. The Bunter conglomerates of the 

 Midland Counties, with which I am very familiar, only differ from 

 some of those in Scotland in the greater prevalence of quartzite 

 pebbles ; this, however, is merely a case of the ' survival of the 

 fittest,' for a quartzite is one of the most durable of rocks. 



We are, I think, justified in looking to a distant locality for this 

 Boulder; because, had it come from near, pebbles of considerable 

 size would be common in the Coal-measure sandstones, which is not 

 thecase in Central England; indeed, I am not aware of any instance 

 either in the Carboniferous or Permian periods (for the breccia of 

 the latter is no parallel) where quartzite Boulders of this size have 

 been observed. I should therefore suppose that it had been brought 

 during a flood from some deposits to the N.W., similar to those 

 which still remain in more than one place about the Firth of Clyde. 

 It might, indeed, have come, like the Bunter pebbles, from some Old 

 Eed Sandstone district to the east of the present Lammermuirs ; but, 

 as the Pennine chain did not then exist, and as the streams, judging 

 from Mr. Hull's curves of thickness,^ ran in a south-easterly direc- 

 tion, it is diificult to understand how anything could have drifted 

 across the line of their current to the position of this Boulder. 

 Both explanations have their difficulties; indeed, with our present 

 knowledge, we can do little more than state the problem, and leave 

 its solution, like that of many other kindred investigations, to the 

 future. 



II. — On " Blocky " EocK Surfaces, akd the Theory of the 

 Shrinking Nucleus of the Globe. 

 By G. Poulett Scrope, F.R.S., F.G.S., etc. 



THE article by Mr. Clifton Ward upon "Eock Fissures" in the 

 Geological Magazine for June, has reminded me of a 

 special characteristic of some igneous rocks, which has been perhaps 

 as yet imperfectly appreciated. I mean by igneous rocks, such 

 masses of subterranean mineral matter as have apparently thrust 

 themselves upwards to the surface of the globe by a process of 

 internal expansion or intumescence, caused by heat — not always 

 attaining absolute fusion, but in some degree owing to the general 

 permeation of the matter by super-heated water, or steam, or other 

 gases. 



The characteristic I refer to is the superficial fissuring of these rocks 

 in the manner aptly called by Mr. C. Ward "blocky"; the surface 

 of the rock or mountain, as in one of the examples described by him, 

 viz. the summit of Scafell Pikes, " being thickly strewn over with 

 large rough blocks of stone, so that one's progression is often limited 

 to jumping from rock to rock." No words could more exactly de- 

 scribe the conformation of the surface of many erupted volcanic 

 masses, such as the basaltic hills of the Siebengebirge ; the ex- 

 tremities of the basaltic plateaux that reach the valleys of the Borne 



^ Quart. Journ. Geol. Soc. xviii. plate vii. 



