1G8 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



-whole of the crystalline series above noticed is of volcanic 

 origin, and consists of the lava and ashes of volcanoes, which 

 have since been altered into these crystalline schists. A like 

 view of the origin of the similar Huronian rocks of Lake 

 Superior was some years since put forth by Nicholson, and 

 G. M. Dawson has also expressed the opinion that a series of 

 rocks in British Columbia, apparently lithologically identical 

 with these, is of Mesozoic age and of volcanic origin. There 

 is nothing, however, in the chemical or litholocncal character 

 of these rocks in central or in eastern America to support 

 such an hypothesis, nor any good reason for believing in the 

 possible transformation of lavas and volcanic ashes into such 

 crystalline schists. These rocks, at least in the regions first 

 mentioned, and in Europe, are Eozoic strata ; and the various 

 hypotheses of which, under the name of pietre verdi, they 

 have been the subject in Italy for the last half century, are 

 instructive in this connection {Record for 1876, page c). From 

 these ancient crystalline rocks, we pass, by a natural transi- 

 tion, to the 



PRE-CAMBRIAN ROCKS OF WALES. 



Rising from below the Lower Cambrian (Harlech) strata 

 at St. David's, in South Wales, is a narrow ridge of crystal- 

 line rocks, which was described by the geological survey of 

 Great Britain as partly intrusive and partly altered Cam- 

 brian strata. Later studies bv Hicks and Harkness have, 

 however, shown that these crystalline rocks are marked by 

 bedding-planes, and belong to two unconformable stratified 

 series, the upper of which contains pebbles of the lower, 

 while the basal beds of the unconformably overlying Cam- 

 brian present a conglomerate containing portions of both of 

 these older series, which were clearly crystalline rocks before 

 the deposition of the Cambrian. To the lower crystalline 

 series Hicks has given the name of Dimetian, from Dimetia, 

 an ancient kingdom including this part of Wales; and to the 

 upper that of Pebidian, from Pebidiauc, the Welsh name of 

 the hundred or district. The Dimetian rocks, which have a 

 northwest strike, are nearly vertical in attitude, and have an 

 estimated thickness of 15,000 feet. They are described as 

 quartzose and feldspathic schistose strata, often greenish 

 and purplish in color, with unctuous surfaces, besides thin 



