No. 5.] HUNT — PRE-CAMBRIAN ROCKS. 267 



the reader is referred to a volume published in 1878 by the 

 Second Geological Survey of Pennsylvania, being Part I of the 

 writer's report on Azoic Rocks, intended as an historical intro- 

 duction to the subject. 



III. — The History of Pre-Cambrian Eocks in 



Great Britain. 



In an address before this Association in 1871, in which the 

 writer maintained the Euronian age of a portion of the crys- 

 talline schists of New England and Quebec, he further expressed 

 the opinion, based in part upon his examinations at Holyhead in 

 1867, and in part upon the study of collections in London, that 

 certain crystalline schists in North Wales would be found to 

 belong to the Huronian series. The rocks in question were by 

 Sedgwick, in 1838, separated from the base of the Cambrian, 

 as belonging to an older series, but were subsequently, by Dela- 

 beche, Murchison and Ramsay, described and mapped as altered 

 Cambrian strata, with associated intrusive syenites and feldspar- 

 porphyries. 



In South Wales, at St. David's in Pembrokeshire, is another 

 area of crystalline rocks, which the geological survey of Great 

 Britain had mapped as intrusive syenite, granite and felstone 

 (petrosilex-porphyry) having Cambrian strata converted into 

 crystalline schists on one side, and unaltered fossiliferous Cam- 

 brian beds on the other. So long ago as 1864, Messrs. Hicks 

 and Salter were led to regard these granitoid and porphyritic 

 rocks as pre-Cambrian, and in 1866 concluded that they were 

 not eruptive but stratified crystalline or metamorphic rocks. 

 After farther study. Hicks, in connection with Harkness, pub- 

 lished in 1867, additional proofs of the bedded character of these 

 ancient crystalline rocks, and in 1877 the first named observer 

 announced the conclusion that they belong to two distinct and 

 unconformable series. Of these, the older consisted of the 

 granitoid and porphyritic felstone rocks, and the younger of 

 greenish crystalline schists, the so-called Altered Cambrian of 

 the official geologists ; both of these being overlaid by the un- 

 doubted Lower Cambrian (Harlech and Menevian) of the region, 

 which holds their ruins in its conglomerates. To the lower of 

 these pre-Cambrian groups, Hicks gave the name of Dimetian, 

 and to the upper that of Pebidian. The last, with a measured 

 thickness of 8000 feet, he supposed to be the equivalent of the 



