PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



(Mem. Geol. Surv. vol. iii. p. 8), that the oldest rocks in Wales and 

 Shropshire appear at the surface in six districts — elevation, denuda- 

 tion, and relative changes having caused to be exposed as six inland 

 islands as many masses, chiefly mountainous, and composed of the 

 oldest sedimentary strata, in Britain, whose true history has 3 r et 

 to be written. In Ireland we may recognize three or four such 

 areas also. 



These and others, of the history of whose masses we are still 

 ignorant, constitute the basis and foundation (mostly unfossilife- 

 rous) of all succeeding sedimentary strata. They have been of late 

 and still are being subjected to critical examination, both chemically 

 and microscopically. Large areas hitherto believed to be of igneous 

 origin, and long ago mapped as such during the progress of the 

 Geological Survey, are now found under rigorous examination to 

 be sedimentary rocks which have undergone complete metamor- 

 phosis since deposition. Such determinations are all-important 

 when we come to consider the earliest appearance of life and its 

 distribution within the British area ; for recent investigation has 

 resulted in the removal of certain rocks, hitherto believed to be 

 igneous, to the sedimentary or stratified series. This has been the 

 case, as we have seen, with several important masses in North and 

 South Wales, now designated Pre-Cambrian, assuming them to be 

 a group of rocks of higher antiquity than those in which undoubted 

 traces of life have hitherto been found in Britain. 



To the influence of microscopic investigation and research is due 

 the right determination and history of these and other doubtful 

 rocks. Dr. Hicks has described some areas in the Lleyn peninsula 

 along the N.AY. shore which he believes to be representative of his 

 Dimetian, Arvonian, and Pebidian groups ; and Professor Bonney 

 doubts not, from microscopical examination, the presence here of a 

 considerable Pre-Cambrian series, with at least two very distinct 

 groups of rocks. 



Dr. Hicks during the past summer determined the presence of 

 an older series of rocks than the Cambrian of the Harlech or 

 Merioneth anticlinal near the centre of the Harlech Dome. These 

 rocks evident!}- underlie the Harlech sandstones, and constitute 

 part of a pre-existing formation. The Cambrian conglomerates at the 

 base of the Harlech grits contain fragments of rock identical with 

 this older formation. 



No discovery of late has equalled this in importance ; it has proved 

 the existence in North Wales of a group or system hitherto un- 

 known, although expected or anticipated. This enables us to compare 

 the thickness of the Cambrian rocks of North Wales with that of 

 those of the same age at St. David's in South Wales, and at the same 

 time to realize and compare the physical conditions of the two. This 

 may enable us to measure the thickness of the Harlech Cambrians, 

 and probably to arrive at the strike of the subjacent rocks. The 

 broken and denuded anticlinal has exposed these Pre-Cambrian 

 rocks, north and south ; they are marked in the maps of the Survey 

 as intrusive felstones, but appear to be a highly metamorphic 



