KiLROE — Silurian and llefamorphic Rocks. 145 



mudstones and limestones, which, at Achill Head at least, show no 

 indications of a steep, shelving littoral. It may he that the houlder- 

 deposit at the base of the quartzite group, though probably the result 

 of glaciation in those far-off times, ^ marked the introduction of a 

 great change of natural conditions resulting generally in the deposition 

 of arenaceous materials throughout the north-west British area, and 

 marked here in Achill by the grit-conglomerate deposit. 



We seem to get further indications of not very distant land of the 

 Dalradian epoch, in the massive bands of fine conglomerate which, in 

 large part, form the small island Achill Beg, where, moreover, are to 

 be seen gray and black mica-schist, some graphitic; and dolomite 

 and steatite after limestone. The conglomerates contain pebbles of 

 quartz, quartzite, and pegmatite ; and both they and the mica-schist 

 show evidence of contortion and over-folding, as well as of intense 

 cleavage and shearing. This may be judged from the accompanying 

 sketch, taken near the southern extremity of the island. 



Still another area of mica-schist, exposed in a hollow between 

 quartzite hills, and fringed as usual by a limestone-bearing zone, is 

 to be seen around Feeagh Lough, north-west of Newport. It is 

 traversed by important basic igneous intrusions which do not here 

 call for special notice. Here, again, we find interesting evidence for 

 the succession of the groups — viz., mica-schist, an intermediate zone 

 with limestone, and quartzite above, though at one point, west of 

 the lake, the regular order of succession is reversed, as shown in 

 the section, fig. 9, on p. 146. 



We have thus seen that the persistence of a fairly well-defined 

 zone throughout this tangled metamorphic series is the key to its 

 true interpretation, preventing us attaching much weight to the 

 present local dip and strike of the rocks, and enabling us to judge of 



^ As suggested by Mr. McHenry : Explan. Mem. of Sheets 3, 4, 5, 9, &c., 

 pp. 17-50. 



R.I. A. PROC, VOL. XXVI., SEC. B. Q 



Fig. 8. — Contorted conglomerate bands in mica -schist. 



