L'ss ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



lations. The lithological characters of the oldest series, 

 which received the name of Dimetian, were at first some- 

 what vaguely given. It may be described as consisting es- 

 sentially of a somewhat granitoid gneiss, generally without 

 mica. What were called quartzites are really very quart- 

 zose feldspathic rocks. Interstratified with this series at St. 

 David's are several thin bands of a coarsely crystalline lime- 

 stone, mixed with quartz and a serpentinic mineral. The 

 exposures of these rocks are comparatively small. Hicks at 

 first included in the Dimetian great masses of what had been 

 by the geological survey of Great Britain described as an 

 intrusive feldspar-porphyry, which, associated with granite 

 (Dimetian), had been injected among the Cambrian strata, 

 locally changing them into the crystalline Pebidian schists. 

 These porphyries, which are found in each one of the four 

 districts, consist of a great series of highly inclined beds of a 

 compact petrosilex or orthofelsite, at times laminated, some- 

 times concretionary or spherulitic, often including crys- 

 tals of feldspar and of bipyramidal quartz (as described 

 by Tawney), and undistinguishable from the halleflinta of 

 Sweden and the feldspar-porphyries so widely displayed in 

 North America {Record for 1876, p. xcvi). These latter 

 were originally regarded, like those in Wales, as eruptive, 

 but were by Hunt declared to belong to a stratified series 

 included by him in the base of the Huronian, and subsequent- 

 ly distinguished by Hitchcock under the name of Lower Hu- 

 ronian. They are absent in many localities in North Amer- 

 ica, between the Laurentian and the Huronian, with which 

 latter the Pebidian of Wales is apparently identical ; and 

 near St. David's, in Pembrokeshire, a conglomerate at the 

 base of the Pebidian includes, in a paste of greenstone or 

 diorite, fragments and pebbles of the banded orthofelsite- 

 porphyries of the vicinity. Elsewhere in that region these 

 rocks are wanting, and the Dimetian granitoid rocks are di- 

 rectly followed by the Pebidian. Hicks, therefore, now re- 

 fers the feldspar-porphyries to an intermediate series, distin- 

 guished by the name of Arvonian, from Arvonia, the ancient 

 Roman name for Caernarvon. The Pebidian rocks of the va- 

 rious areas named are undistinguishable in lithological char- 

 acters from the Huronian of North America, as seen alike in 

 the Atlantic belt, near the great lakes, and in the region of 



