OBEEMITTWEIDA CONGLOMERATE. 31 



even rather fewer symptoms of having been squeezed than I should 

 have expected from the macroscopic aspect of the blocks. 



The amount of metamorphism in the rock is more than I have been, 

 accustomed to see in Palaeozoic grits *, or even in those generally 

 supposed to be a little older than the Cambrian series f. I should 

 conjecture that the Obermittweida conglomerate was Archaean, but 

 should not assign it to a remote period in that series. Still, in the 

 present state of our knowledge, it would be unsafe to do more than 

 conjecture. Be this as it may, I think it far more likely that 

 the conglomerate beds are much newer than the gneiss, mentioned 

 in the beginning of this paper, than that they are in true chronological 

 sequence with it. I have had the opportunity of examining, both in 

 the field and with the microscope, cases of these supposed sequences, 

 and bave always found the appearance of transition to be illusory, due 

 to the fact that the later formation is so largely composed of frag- 

 mental material derived from the earlier, and that the effects of subse- 

 quent pressure have obscured the break between them, have produced 

 sensible modifications of both rocks, and so have brought about a delu- 

 sive appearance of forming parts of one and the same group of strata. 



(Por the Discussion on this paper, see p. 45.) 



* I do not here take into consideration quartzites like that of Loch Maree ; 

 for a clean sandstone is evidently readily changed to a quartzite, but the de- 

 velopment of mica, other than the most microscopic, is less easy. 



t I refer to the sedimentary rocks underlying the basement pebble-beds of 

 the Cambrian in North and South Wales. 



