30 HOW TO WORK IN THE ARCH^AN ROCKS. 



HOW TO WORK IN THE ARCH^AN ROCKS.* 



BY C. CALLAWAY, M.A.. B.SC. (LOND.,) F.G.S. 



The ArclifBan (Pre-Cambrian) rocks have recently excited consider- 

 able interest, owing partly to the more or less complete working out of 

 the yoianger groups, and partly to the fascination which attends a 

 study of peculiar complexity. 



In America, six Archsean systems have been described, which, taken 

 in descending order, are the following : — 



I. — Kciccenian, or copper-bearing series of Lake Superior. 

 II. — Taconicui. 

 III. — Montalban, or mica-schist series. 

 IV. — Huronian. 



V. — Norian. 

 VI. — Laurentian. 



In Britain, Murchison recognised the Laurentian in the great gneiss 

 series of the Hebrides, and Dr. Holl claims the same antiquity for the 

 Malvern ridge. Salter and Hicks discovered two Archaean groups, 

 Dimetian and Pebidian, at St. Davids. Two Archcean formations have 

 also been recognised in Carnarvonshire, and the writer has worked 

 out two groups in Anglesey. He has also discovered two series, a 

 volcanic and a metamorphic, in Shropshii-e, and has detected the 

 former on the flanks of the Malverus. The slaty and brecciated rocks 

 of Charnwood Forest have also been referred to the Archsean by Dr. 

 Hicks, subsequent to their description by Messrs. Hill and Bonney. 



Notwithstanding the peculiar difficulties attending the study of 

 these rocks, there is no reason to despair of success, and, in this paper, 

 the methods of work are indicated. 



The evidence of organic remains is rarely applicable. The organic 

 nature of Eozoon is strenuously disputed, and, in the present state of 

 the controversy, the (so-called) fossil is of little value as a test of age. 

 Besides this, a similar structure has been discovered in the Taconian, 

 and Murchison even claimed it as a Silurian fossil. The traces of 

 annelides, which are found in some very ancient rocks, are hardly 

 distinguishable from recent tracks and burrows, and are of little classi- 

 ficatory use. 



The test of order of sjqyerjMsition is frequently complicated by 

 inver.-iioiiK. In North Ainericathe Archsean rocks have a general south- 

 east dip, but really they are made up of numerous parallel folds, with 

 their summits thrown over to the north-west. The contorted schists 

 of Anglesey display the same phenomena. The contortions have been 

 disentangled by the discovery by the writer of a grey gneiss underlying 

 the prevailing green schist, and the latter is seen to lie in sharp 

 synclinal folds between anticlines of the gneiss. 



The superposition test is also complicated by faulting. The 



Summary of a paper read before the Society, on June 28th, 1881. Published 

 in full in the " Geological Magazine." 



