Vol. 56.] SILURIAN SEQUENCE OF RHAYADER. 95 



(d) Additional Notes on Supplementary Exposures. 



Before summarizing my observations of the Gwastaden Group 

 as a whole, it is necessary to fill up some important gaps in the 

 typical sections. The missing beds are found in several minor 

 exposures, which occur between the typical section-lines already 

 described, but they have not been mentioned hitherto lest our 

 minds should be confused as to the general order of the beds. 

 Now that this general order has been established, I turn to the 

 description of these minor sections, with the view of giving the 

 complete succession, from bottom to top of the series. 



(1) Cerig Gwynion Grits. 



This group (Aa), as I have already stated, is most completely 

 exhibited in the Cerig Gwynion Crags, after which the beds are 

 named. At the foot of these crags a quarry was opened for 

 supplying a part of the Birmingham Waterworks with building- 

 stone, and an interesting section is here exhibited. In the quarry 

 itself the group has a total thickness of over 230 feet. The typical 

 rock, which is found in beds running up to 10 feet in thickness, is 

 here a very dense, tough, hard quartzose grit or grauwacke. The 

 colour varies from bluish-grey to greenish-grey. Occasional beds of 

 coarse rock, with white quartz-pebbles about the size of a pea, are to 

 be found on certain horizons, and a lenticular bed of conglomerate 

 occurs at about two-thirds of the thickness up the group. This is a 

 somewhat calcareous bed, with pebbles averaging about 1 inch in 

 diameter. Its maximum thickness in the quarry is only about 

 2| feet ; but it is a remarkably persistent bed, and may be traced 

 westward on the opposite side of the river for a distance of over 

 \\ miles. 



'Thin dark-blue slates interbedded with the grits form well- 

 defined platforms in the cliff, giving a rugged and scarped ap- 

 pearance to its face. These bands yield Glimacograptus normalis, 

 and this species and its variations only : no traces of Diplograptidse 

 or MonograptidaB having been detected by me at any time. Micro • 

 scopic sections of the grits show the constituent grains to be sub- 

 angular or rounded in form, and to consist mainly of quartz. 

 Fragments of vein-quartz and felspar are not uncommon ; more 

 rarely bits of felsite, microgranite, andesite, basic rocks, and slates 

 occur. Mica may be found in some quantity in certain beds, but 

 it forms by no means a common constituent of the rocks, considered 

 as a whole. 



As the group is followed westward, we have already seen that it 

 rapidly thickens out to more than twice its normal thickness at 

 Cerig Gwynion. This augmentation may be due partly to strike- 

 faulting, but only in a small degree ; for complete sections through 

 the group fail to show any repetition of the beds. To the eastward 

 the conditions are reversed, for the group thins down to something 

 like 100 feet at the eastern limit of the district. The individual 



