270 THE CANADIAN NATURALIST. [Vol. ix. 



side by Pebidian and Cambrian, and on the southeast by Silurian 

 strata, let down by a fault. 



On the shore of Llyn Padarn, near the foot of Snowdon in 

 North Wales, the porphyritic petrosilex of the Arvonian is again 

 well displayed, while in contact with it, and at the base of the 

 Xlanberris (Lower Cambrian) slates, is a conglomerate made up 

 almost wholly of the petrosilex. This locality was supposed by 

 Prof. Ramsay and others to show that the petrosilex is the result 

 of a metamorphosis of the lower portion of the Cambrian, the 

 conglomerates being regarded as beds of passage. The writer, 

 after a careful examination of the locality, agrees with Messrs. 

 Hicks, Hughes and Bonney that there is no ground for such an 

 opinion, but that the conglomerate marks the base of the Cam- 

 "brian, which here reposes on Arvonian rocks, and is chiefly made 

 Tip of their ruins. In like manner, according to Prof. Hughes, 

 the Cambrian in other parts of this region includes beds made of 

 the debris of adjacent granitoid rocks. 



These petrosilex-conglomerates of Llyn Padarn are indistin- 

 guishable from those found at Marblehead and other localities 

 near Boston, Massachusetts, which have been in like manner in- 

 terpreted as evideoces of the secondary origin of the adjacent 

 petrosilex beds, into which they have been supposed to graduate. 

 The writer has, however, always held, in opposition to this view, 

 that these conglomerates are really newer rocks made up of the 

 ruins of the ancient petrosilex. He has found similar petrosilex- 

 conglomerates at various points on the Atlantic coast of New 

 Brunswick, of Lower Cambrian, Silurian and Lower Carbonifer- 

 ous ages, all of which have, in their turn, been by others regarded 

 as formed by the alteration of strata of these geological periods. 

 The evidence now furnished in South Wales of still older 

 (Huronian) beds of petrosilex-conglomerate should be noted by 

 students of North- American geology. From observations near 

 Boston, made by one of my former students, I have for some 

 time suspected the existence of petrosilex conglomerates of Pre* 

 Cambrian age. 



To the eastward of the localities already mentioned in Wales, 

 are some other small areas of crystalline rocks, including those 

 of the Malverns, and the Wrekin and other hills in Shropshire, 

 all of which appear as islands among Cambrian strata ; also those 

 of Charnwood Forest, in Leicestershire, which rise in like manner 

 among Triassic rocks. The Wrekin, regarded by Murchison as 



