Vol. 56.] SILURIAN" SEQUENCE OF RHAYADER. 101 



never be confused in the field. The total thickness cannot be 

 estimated at this point, for a cross-fault cuts out some of the topmost 

 beds : it is probably not less than 150 feet. 



Intermediate Shales (Ba 2 ). — Succeeding the Caban Con- 

 glomerates is a thick mass of highly cleaved, micaceous blue 

 shales (Ba 2 ). Only a few scattered patches, unfortunately, can be 

 seen. They show the shales to be blue-banded and occasionally 

 striped with thin arenaceous layers. The shales weather to a brilliant 

 red, a coloration which forms a characteristic feature of the group. 

 The thickness is between 200 and 300 feet. 



Upper Conglomerate (Ba a ). — The shales are followed by a 

 second conglomerate-and-grit group. This differs from the lower 

 conglomerate, chiefly in the predominance of white vein-quartz- 

 pebbles. The larger fragments are, as a rule, of grit and quartzite : 

 the smaller being of felsite. There seems to be, in addition, a 

 certain amount of manganese in the composition of the matrix, 

 which gives rise to a pronounced purplish-brown weathering at 

 certain horizons. Fragments of various iron-ores also may be 

 seen in microscopic sections of the rock. The thickness at this 

 point is about 100 feet. 



(2) Gafallt Beds (Bb). 



Jlonograptus-Sedgwiclcii Grits (Bb r ). — The next division 

 is remarkable for the extraordinary contrast which it presents with 

 the underlying group. It may be described as a regularly alternating 

 series of fine-grained grey flags and smooth pale grey-and-green 

 shales. The grits contain a fairly high percentage of felspar, in ad- 

 dition to the quartzose material. They are striped with carbonaceous 

 bands, and in places appear to be false-bedded. Commencing with a 

 thickness of about 2 feet, they rapidly thin out to an average of 

 from 4 to 6 inches. A distinctive feature is the pronounced red- 

 stained appearance of the beds on their exposed surfaces. A thin 

 puddingstone-bed occurs near the base, and is made up largely of 

 white quartz-pebbles. The chief peculiarity of the group as a whole 

 is the regular alternation of grits and shales, and the parallelism 

 of the upper and lower surfaces of the individual beds. Crinoid- 

 rings are not uncommon in the grits themselves, and graptolites are 

 met with in certain carbonaceous bands ; but they are, at this 

 particular locality, too badly preserved to admit of identification. 

 At about 200 feet above the base of this division occurs a pudding- 

 stone-bed of considerable thickness, which forms a convenient 

 horizon for separating this division from the succeeding one. 



Gafallt Shales (Bb 2 ). — The cap of the hill is formed of a basin 

 of pale grey and greenish-grey shales. Thin bands of sandstone 

 or grit occurring throughout the group mark its affinity with the 

 underlying division ; but, considered as a whole, it is a shale-group 

 as distinct from a series of grits. The harder bands are always 

 felspathic, often ferruginous; and they generally decompose into 

 soft rottenstones which rarely exceed 2 inches in depth. Another 

 feature is the presence of worm-burrows preserved in orange-coloured 

 limonite. The vertical thickness exposed is only about 50 feet. 



