﻿Vol. 66.~\ TREMAD0C SLATES OF SOUTH-EAST CARNARVONSHIRE. 151 



of the bedding -within the nodules involves curvature in three 

 dimensions ; but, although the actual nodules in the pavements seem 

 to have a directional elongation due to the later cleavage-stress, I 

 am unable to discover any polarity in the actual arrangement of 

 the curled bedding-planes. A certain amount of discontinuity 

 • and some evidence of sliding is usual between a curled-bedded 

 grauwacke and the super- or subjacent flag or slate. 



Under the microscope the curled-bedded grauwackes have little 

 "to distinguish them. They contain much fresh felspar, and possibly 

 the recrvstallization of the quartz-grains and quartz-cement is more 

 ■advanced than in the evenly-bedded flags, but I have not examined 

 enough specimens of either sort to be sure. It is the quartz-cement 

 and its recrvstallization in crystalline continuity with the quartz- 

 .grains which makes the rocks so tough. 



I have measured several sections across the moorland exposures 

 •of the Ffestiniog Beds, but cannot discover either fossil bands or 

 well-characterized lithological groups within them, and have there- 

 fore had to rest content with making a strike-map of their exposures 

 (see map, PI. XVII). The most open section is through the farm 

 of Ty'n-y-mynydd on the south side of Moel-y-gest, where the 

 total thickness exposed is about 1700 feet. In this section the 

 rocks appear to fall into five groups, each marked by a convexity 

 on the hillside, and having a flat or hollow separating it from its 

 neighbour. 



The lowest group (about some 400 feet thick) consists of rather 

 •dark-grey flags with thin and thick beds alternating. These, from 

 •a distance, show some eight or nine subsidiary features, with the 

 hard bands (JSTos. 5 & 6) most prominent. 



The second group is more massive, and has but few and very thin 

 bands well bedded enough to be called ' flags.' It also is nearly 

 400 feet thick, and might be further subdivided according to some 

 four or five grit maxima. 



The third group, about 300 feet thick, is less massive in its bedding, 

 and consists of thickly-bedded flags with not infrequent Liagiilella. 

 It would subdivide into either three or five, by the grouping of its 

 resistant bands. The house Ty'n-y-mynydd stands on the ledge at 

 the top of this third group. 



The fourth group begins Avith a set of very strong, thickly-bedded, 

 grey-weathering, massive grey grits, wdth grey flag-partings. These 

 gradually diminish in weather-resisting power upwards, and pass 

 into grey flags. They are about 250 feet thick, and might subdivide 

 into five. 



They pass gradually upwards into the fifth group, which consists 

 of flags of a much darker and more rusty colour. These subdivide 

 readily into some seven grit maxima, and are about 350 feet 

 thick. The highest big grauwacke is about 100 feet from the top 

 of the series and 50 feet below the base of the well-marked 

 Lingulella Band, which in this, as in the Arenig district, is very 

 distinctive. 



