O.N TIIK AIICII.KAN ItOCKM OK CHEAT IIUITAIX. 



5:57 



apparently, are sandry slaiy bods nnd a few bands of a pebbly quartz-grit 

 and quartzitc, over which, in the sonthern region, are the well-known 

 workable slates of Swithland and Groby. 



Thus from first to hist volcanic materials are recocfnisable, 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 71) per cent, of silica. Jfc is dithcult 

 to decide upon the true nature of the Shorpley 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 

 niicro-mineralogical changes which have occurred help to increase the 

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

 oriji^iii ; but the dilliculty is wliethor we should regard them as lavas 

 orii,'inally glassy, upon which a rude cleavage has been imprc^^sod, and 

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

 character, or as tuff's of similar chemical composition, indurated, cleaved, 

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

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

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

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

 Ardeimes ])orpbyroids (some of which closely resemble the Sharpley 

 roek), has made me more sensible of the difficulties of tliis hypothesis, 

 and removed some of those in the other view. At the same time 1 

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

 particular rocks, with a little of tliose at Bardon Hiil, be sheets of lava 

 or not, a very largo portion of the Ciiarnwood Forest series, like tins 

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

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

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

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

 replaced by various minute products of secondary orii^in ; augitic orhorn- 

 blcndic minerals have been replaced by ' viridites ' ; in the porphyroids 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

 ori<:;inal 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 qmirtz, are as clear as in many volcanic deposits 

 of Ordovician ' age. The very local 'contact nietamorphism ' 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 

 onlv ai'isinjT from the minute and indefinite character of the rock struc- 

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

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

 Silurian. 'J'he reason for assigning it to the latest Archiean rather than 

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

 tioned hereafter. 



The Litnisice Igncnni liorls. — (a) A. mass of hornblendic granite at 

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

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



'I I 



I- 

 I 



' I nilo],) this term, proposed by I'rof. Lapwortli to include the beds from the base 

 (lE the Arciii^- to tlie base of the Upper Llandoverv, to avoid the ambiguity of the 

 Ijimndiny of the Lower Silurian. 



