PRECARBONIFEROTTS ROCKS OF CHARNWOOD FOREST. 



235 



wood rocks, and in this neighbourhood, in Postcarboniferous times, 

 sheets of basalt were erupted over the Coal-measures. 



Age of the Clastic Charnwood Rocks. 



One of the writers has more than once called attention to the 

 close resemblance which some of the Charnwood rocks present 

 macroscopically to those of the Borrowdale series in the Lake district. 

 This similarity becomes even more striking when both are studied 

 microscopically. He has had the opportunity of examining some 

 of Mr. Clifton Ward's collection of slides used in his important 

 paper (Q. J. G. S. vol. xxxi. p. 388), and has also had a few pre- 

 pared for comparison from specimens in his own collection. Two 

 of these, altered ashes from the flank of Dod's Pike and the west 

 side of Three-shire Stone, might readily be supposed to have come 

 from such a district in Charnwood as the vicinity of the Monastery. 

 The fragmentary felspar crystals and lapilli can be distinguished, 

 and there are the same peculiarities of the matrix ; quartz, how- 

 ever, in these happens to be wanting *, though it is present in other 

 varieties. The Charnwood rocks have also a considerable similarity 

 to the " porphyroids " of the Ardennes described by MM. Poussin 

 and Renard f . There the same minerals are stated to occur — quartz 

 with endomorphs, felspar of all kinds, fibrous hornblende, epidote, 

 both ordinary viridite and the associated serpentinous mineral, 

 and decomposed ilmenite. Many of their descriptions might be 

 taken word for word, and the plates might be copied, to represent 

 some of the structures of the Charnwood rocks. They have the 

 same difficulty about the included crystals of quartz and felspar, 

 and are of opinion that while many are certainly clastic, some may 

 be formed in place. These Belgian rocks are considered to be of 

 Lower Silurian age. 



In the Survey Memoir and Map, as by most authors, the Charn- 

 wood rocks are called Cambrian, the term being used in the 

 restricted sense in which it was employed by Murchison. The only 

 evidence in favour of this correlation is that the Charnwood rocks 

 are certainly old and unfossiliferous, and that they have an unusual 

 strike, which, however, is not characteristically Cambrian. The 

 authority of Professors Sedgwick and Jukes is often quoted in 

 favour of their Cambrian age; but, as one of us has more than 

 once remarked, this proves nothing more than that these authors 

 thought them older than the May-Hill beds, seeing that they 

 employed the term Cambrian in a sense so much wider 

 than was afterwards given to it by the Survey. In the absence 

 of palseontological evidence some weight may be rightly given 

 to lithological ; for lithology is a record of physical phenomena, 



* Mr. Ward's analyses show that the ashes contain about 68 or 69 per cent, 

 silica. I should expect the Charnwood would be the same. The lavas seem 

 to be poorer in Si 2 , about 60 per cent. The Charnwood fragments, so far as 

 examiued, are decidedly rich in silica. — T. G. B. 



t ' Memoire sur les roches plutoniennes,' &c. 



b2 



