136 Reports and Proceedings — Geological Society of London. 



The present paper deals in the first place with the stratigraphy of 

 the Northern Complex. The following table shows the succession of 

 the rocks in descending order : — 



C. The Llandeilo Strata. 



Feet. 



C^. Black argillaceous shales 40; topnotseen. 



Cc. Ironstone, in part oolitic ... ... ... ... ... 20 



Gb. Grey quartzose shales or slates, striped by thin, black lamina; 150 



Ca. 2. Pale conglomerates and grits ... ... ... ... 500 



1. Eed-purple conglomerate 180 



B. The Llanbadrig Series, about 1,000 or 1,500 feet; apparent order : — 



B/. Quartzite. 



B e. Pebbly slates. 



B d. Slates with grit and quartzite bands. 



Be. Quartzites, shales, and some limestone (Perth Padrig, etc.). 



B b. Limestone (as at Trwyn y Pare). 



Ba. Grits and slates, usually smashed to a crush-conglomerate. 



A. The Green Series. 



Greenish and bluish slates (of the Northern Complex) . 



The rocks strike east-soufh-east, and dip usually at a high angle 

 northward ; a well-marked transverse fault divides the complex 

 into western and eastern portions. The Llandeilo rocks occupy four 

 strips of ground in the west and three in the east, but the full 

 succession does not occur in any one of these outcrops. The effects 

 of comjjression are much marked in the purple conglomerate ; the 

 matrix is dragged out along small shear-planes, and the pebbles are 

 stretched into phacoids, slip-faulted, and their extremities tailed out. 

 A deceptive appearance of unconformity is also produced at the 

 junction of grits and shales. Fossils are found in some of the 

 rocks. The Llandeilo rocks are of importance, as they reveal the 

 existence of at least four shattered synclines, usually faulted, and 

 probably all overthrust on their northern boundaries. 



No fossils have been found in the Llanbadrig rocks, and no more 

 definite assertion of their age is possible than that they are pre- 

 Llandeilo. They are newer than, and their base may be con- 

 formable with, the Green Series ; but as fragments almost certainly 

 derived from, the latter series have been found in the highest zones, 

 the relation between the two series is more likely to be that of an 

 unconformity. The Llandeilo rocks contain fragments of limestone 

 and pieces of quartzite; but in spite of this evidence in favour 

 of unconformity between the Llanbadrig and Llandeilo rocks, the 

 basal beds of the latter are found to cling closely to the highest 

 quartzite of the Llanbadrig Group. 



The quartzites are remarkable in the fact that they frequently 

 contain patches of limestone, apparently deposited with them, and 

 that they vary rapidly in thickness from about 30 to 200 feet 

 at Craig Wen, where this variation is correlated by an inverse 

 variation in the conglomerate. Although earth-movement may be 

 partly responsible for this variation, some further explanation 

 appears requisite, and this may possibly be furuislied by erosion of 



