348 Dr. C. Callaicay — On the Archcean Rochs. 



II. — How TO WOKK IN THE ArCH^AN KoCKS. 

 By C. Callaway, M.A., D.Sc. (Lond.), F.G.S. 



THE Arch^au (Pre-Cambrian) rocks of Britain have of late years 

 received considerable attention, owing partly to the more or 

 less coinplete working out of the younger groups, and partly, 

 perhaps, to the fascination which attends a study of peculiar com- 

 plexity. "Whatever interest attaches to the correlation of formations 

 which can be easily identified by their organic remains, or which 

 can be traced across country for scores of miles in unbroken lines, 

 it will be readily understood that the spirit of inquiry will be 

 strongly piqued when it is challenged to construct orderly systems 

 out of rock-masses to which the ordinary tests can be but partially 

 applied. Yet much has been done towards the establishment of a 

 succession amongst these ancient groups. In America, Dr. Sterry 

 Hunt describes six distinct systems. These, taken in descending 

 order, are the following : — 



I. Keweenian, or Copper-beariDg series of Lake Superior. 

 II. Taconian. Perhaps the same as I. 

 III. Monfalban, or Mica-schist series. 

 IV. Suronian, or Green Mountain series. 



V. JYorian, or Labradorian. 

 VI. Laurentian. 



Some of these groups are undoubtedly established, while others 

 are not undisputed ; but should this succession, or the major part of 

 it, be ultimately received, it is evident that we shall have, under- 

 lying the " bottom rocks " of Murchison, a group of rock systems of 

 at least equal value to any of the three great divisions : — Palaeozoic, 

 Mesozoic, and Cainozoic. 



The study of these old groups has also made considerable progress 

 in the British Islands. Murchison recognized the Laurentian in the 

 great gneiss series of the Hebrides and the north-western Highlands, 

 and Dr. Holl claims the same antiquity for the metamorphic ridge 

 of the Malvern Hills ; but the discoveries of Mr. Salter and Dr. 

 Hicks at St. Davids have given the chief stimulus to these investi- 

 gations. The two recognized formations worked out by Dr. Hicks, 

 the Dimetian and the Pebidian, furnish us with a clue to unravel the 

 complexities of some other districts. The probable equivalents of 

 these groups have been identified in Caernarvonshire. Anglesey is 

 mainly occupied by two Archaean series, one of which is clearly 

 Pebidian, while the other, a great gneissic formation, may jDOSsibly 

 be Dimetian. The Malvern gneiss group has been identified in the 

 Wrekin, while the same mountain and many other Shropshire hills 

 are mainly built up of bedded lavas and ashes which may be 

 Pebidian, and are certainly Archeean. Pebidian rocks occur in the 

 Herefordshire Beacon, near Malvern ; and the Charnwood slaty 

 series probably belongs to the same group. 



The difficulties attending these investigations are so great that 



