﻿622 ME. W. G. PEARNSIDES.ON THE GEOLOGY OP [Aug. 1905, 



in the basal Ordovician Grit of Rhobell Fawr. The upper band 

 of ash is thinner but even more gritty ; and it must, I think, have 

 been the character of this rock which led Miss Elles l to conclude ■ 

 that the Hirundo-'Be&s at Nant Khos-ddu rest unconformably upon 

 Tremadoc rocks, and to record ashes as occurring in the latter 

 group. Above the higher ash the beds are again very blocky, and 

 so calcareous that they were termed limestones by Miss Elles, 

 who obtained Didymograptus hirundo, Salt., from the layer directly 

 above the ash-band. In general, the Ilirundo-Beds are much softer 

 than the Erwent Beds below, or than the volcanic group which 

 covers them with screes from above. Exposures are correspondingly 

 unsatisfactory, and it was only where a small landslip had bared a 

 scar on the slope of Maen Grugog, close to Filltirgerig, that I was 

 able to see the main mass of the shales in situ. Fortunately, in 

 that same area the trend of the giaciation coincides for some 

 distance with the outcrop of the beds, which have therefore pro- 

 vided a moderate proportion of the smaller boulders in the adjoining 

 Drift, and over all the slopes above Filltirgerig examples of 

 Didymograptus Jiirundo, Salt., may be occasionally found in the 

 Drift. A very good place to search is the Drift exposed in the 

 banks of the Nant Filltirgerig, immediately above the waterfall. 

 The ash-bands and higher beds are best seen in the JNTant-Rhos-ddu 

 section, but are also easily made out where they cross the Hafotty- 

 Filltirgerig stream. The well-known mass of Ampyx-h earing 

 streaky beds there exposed is probably a fallen block or transplanted 

 boulder, and is quite out of place. 



The Olchfa or Bifidus-Sh&les [8]. 



Upwards the Hirundo -faun a seems to die out suddenly, and 

 within a foot of the bed in which the last Didymograptus 

 hirundo was collected, quite a selection of species belonging to the 

 Bifidus- fauna may be found. Correspondingly the sediments 

 lose their calcareous character, but this variation is gradual ; and 

 were it not that the Bifidus-Beds are pyritous instead of calcareous, 

 the two would be indeed difficult to distinguish. Here, however, 

 fossils come to our aid, and are so abundant that even poor 

 exposures will usually yield determinable graptolites. 



Under the microscope it is seen that the Bifidus-Shales are 

 far finer in texture than those of the Hirundo-gr oup, but they also 

 contain chips of detrital quartz and shreds of mica which are much 

 too coarse for the rest of the materials. The slide also shows 

 many ragged flakes of what is probably graphite, together with the 

 more usual pyrites (4648). 



The Bifidus-Beds, like the soft shales of the Shumardia-gvouip, 

 never seem to be properly cleaved, and, although distorted, the rock 

 always breaks along the bedding-planes, which, under the micro- 

 scope, appear f lightly puckered. The fauna obtained is the typical 



1 Geol. Mag. 1904, p. 207. 



