ON THE ARCHAEAN HOCKS OF GREAT BRITAIN. 537 



apparently, are sundry slaty beds and a few bands of a pebbly quartz-grit 

 and quartzite, over which, in the southern region, are the well-known 

 workable slates of Swithland and Gfroby. 



Thus from first to last volcanic materials are recognisable, often as 

 very coarse agglomerates, the fragments of lava being quite unmistak- 

 able. This is a compact quartz-felsite (old quartz-trachyte or rhyolite), 

 containing in one case as much as 79 per cent, of silica. It is difficult 

 to decide upon the true nature of the Sharpley and the Peldar Tor rocks ; 

 quartz and felspar occur porphyritically in a compact devitrified matrix, 

 which is curiously devoid of any very characteristic structure, and the 

 micro-mineralogical changes which have occurred help to increase the 

 difficulties. It may be regarded as a certainty that they are of volcanic 

 origin ; but the difficulty is whether we should regard them as lavas 

 originally glassy, upon which a rude cleavage has been impressed, and 

 which, owing to subsequent changes, have become slightly schistose in 

 character, or as tuffs of similar chemical composition, indurated, cleaved, 

 and slightly altered, so that the original fragmental structure has been 

 practically obliterated. When I wrote last upon the subject I inclined 

 to the latter view, but prolonged study of these and other rocks of 

 volcanic origin, ancient and modern, together with field work among the 

 Ardennes porpbyroids (some of which closely resemble the Sharpley 

 rock), has made me more sensible of the difficulties of this hypothesis, 

 and removed some of those in the other view. At the same time I 

 would not venture to speak positively, except to say that whether these 

 particular rocks, with a little of those at Bardon Hill, be sheets of lava 

 or not, a very large portion of the Charnwood Forest series, like the 

 Borrowdale group in the Lake country, is of volcanic origin, and the district 

 was probably the site of a number of cones, perhaps individually of no 

 great size. The changes, it must be remembered, are never more than 

 'micro-mineralogical.' The felspar has been somewhat decomposed, and 

 replaced by various minute products of secondary origin ; augitic orhorn- 

 blendic minerals have been replaced by ' viridites ' ; in the porphyroids 

 a minute filmy mineral, possibly allied to sericite, has been produced, and 

 lavas once glassy have assumed a devitrified structure ; but usually the 

 original clastic character of the rock, the structure of the volcanic lapilli, 

 with abundantcrystallites of felspar and some other minerals and with small 

 crystals of felspar and quartz, are as clear as in many volcanic deposits 

 of Ordovician ' age. The very local 'contact metamorphism ' at Brazil 

 Wood is the only instance of important mineral change in situ in the 

 whole region, the difficulties at Sharpley, Peldar Tor, and Bardon Hill 

 only arising from the minute and indefinite character of the rock struc- 

 tures. So far as the evidence obtainable goes, the rock must be much 

 older than the Carboniferous limestone, and is probably anterior to the 

 Silurian. The reason for assigning it to the latest Archnsan rather than 

 to the Cambrian (as it is named by the Geological Survey) will be men- 

 tioned hereafter. 



The Intrusive Igneous Rocks. — (a) A mass of hornblcndic granite at 

 Mountsorrel, which is surrounded by Trias. This, as discovered by Mr. 

 Allport, is intrusive in a slate, probably belonging to the uppermost 



1 I adopt this term, proposed by Prof. Lapworth to include the beds from the base 

 of the Arenig to the base of the Upper Llandovery, to avoid the ambiguity of the 

 boundary of the Lower Silurian. 



