214 



C. CALLAWAY 02? THE AECHJEAN 



a. Slaty Series. 



1. Llangristiolus Grits and Slates. — The lowest beds I have ob- 

 served are exposed near the Llangristiolus turnpike. The rock is 

 clearly bedded ; but its lithology is obscure, and the jointing is so 

 close that clean fractures are hard to get. It appears to be a quartzo- 

 felspathic grit, altered into a sort of halleninta. Some bands are 

 coarser, grains of quartz and felspar being visible to the naked eye. 

 A similar rock is common in the Pre-Cambrian rocks of the Church- 

 Stretton district (Shropshire). 



A little further to the north, hear Cerrig ddwyffordd, a quarry- 

 section is very interesting. At the base is a purplish grey felspathic 

 grit (Note 42, p. 235). This is overlain by a pale-green slaty rock of 

 Charnwood type, associated with, and apparently passing into, a 

 sort of porcellanite (Note 43, p. 234), singularly like a rock at Caer 

 bwdy, St. David's. The Charnwood facies of these varieties is also 

 noted by Prof. Bonney. The section is capped by a repetition of 

 gritty bands. The dip is northerly, at from 40° to 50°. Similar 

 rocks are exposed in neighbouring fields. 



2. Llangefni Conglomerates and Shales. — To the west and north- 

 west of the Llangristiolus sections are several exposures of a greenish 

 conglomerate. It is seen on the Holyhead road, near Waen hir, and 

 north of Cerrig ddwyffordd, at the arrow mark, but is very fully ex- 

 posed in the railway- cuttings north-west of Llangefni, the enclosed 

 fragments, weathering white, being clearly visible even from the rail- 

 way carriage. The matrix is green shale. The pebbles are very 

 varied, the following being the principal types : — quartzite, common ; 

 pinkish grit (Note 38, p. 235) of quartz and felspar ; greenish fel- 

 spathic grit ; and green and grey hornstone. Of these, quartzite is 

 the only ingredient which is certainly foreign to the slaty series ; the 

 others are such as are found in some part of the group. The con- 

 glomerate would then seem to have originated in contemporaneous 

 denudation, such as is common in volcanic rocks, together with the 

 wearing-down of a preexisting land composed of quartzite. 



Interstratified with the conglomerate are beds of purple and 

 green felspathic shale, with some bands of hornstone. The resem- 

 blance of these rocks to the St.-David's types is unmistakable. 



The road-sections in this district display a prevailing north-west 

 dip; but the more complete exposures in the railway-cuttings 

 reveal frequent undulations, so that the thickness is not great. 



A similar group, probably on the same horizon, is well displayed 

 in the cuttings east of Bodorgan station, 6 miles south-west of 

 Llangefni. The prevailing rocks are purple, green, and grey ashy 

 shales, with some felspathic breccias. Conglomerates occur con- 

 taining pebbles of felsite, in which microscopic examination (Note 39, 

 p. 236) reveals a very interesting resemblance to modern lavas. 



Associated with the shales at Llangefni station is a thick bed of 

 quartzose grit, such as might have been produced by the denudation 

 of an ancient quartzite. 



Still following the strike to the south-west, we find the slaty 

 series well exposed at numerous points on the coast from Bodowen 



