Vol. 62. ,] TARAKNOff SERIES OP TARANNON. 655 



The Fynyddog Grits and Nant-ysgollon Shales correspond to the 

 hands of Denbighshire Grit and underlying Denbighshire Shale, as 

 laid down on the Geological-Survey maps and coloured as Wenlock. 

 The fossil-evidence here obtained definitely establishes their 

 Wenlock age, for the fauna of the Nant-ysgollon Shales is that of 

 the zone of Cyrtograptus Murchisoni. which is the basement-zone 

 of the Wenlock elsewhere. Below the Nant-ysgollon Shales we 

 enter upon the beds which belong to the Tarannon Series. 



(2) Tarannon Series (B). 



(i)Dolgau Mudstones (Bd). (Zone of Monograptus 

 crenulatus.) 



Beyond a distance of some 170 yards below the Ford the strata 

 for 200 or 250 yards present distinctive lithological characters, 

 which readily serve to separate them from the ]N"ant-ysgollon 

 Shales above. They consist, for the most part, of fine, smooth, 

 greyish-green mudstones and shales, with occasional thin flaggy 

 beds from about a quarter to 1| inches thick, which become more 

 abundant as we descend the section. They dip steadily north- 

 westward, at an angle of about 40°. 



This group of strata graduates so insensibly from the Nant- 

 ysgollon Shales above, that it is difficult to fix upon any definite 

 boundary between them. In this Tarannon section, however, a 

 convenient separating-line is formed by some few coarse greywacke- 

 bands which give origin to a small waterfall. The Mudstone Group 

 as a whole is divisible into three sub-groups. 



Upper Green Mudstone Sub-group (Bd 3 ). — The beds of 

 this highest sub-group consist of mudstones of a peculiarly-smooth 

 and soapy texture, associated with a few thin flags. When fresh, 

 these mudstones are of a pale greyish-green colour, but they weather 

 to a dirty brown. The rocks occasionally show evidence of movement, 

 and are in places slickensided and wrinkled, but the disturbance 

 does not affect their general disposition. No trace of fossils has 

 been detected in this sub-group. 



Middle Purple Mudstone Sub-group (Bd 2 ). — The green 

 mudstones give place to a second sub-group of strata, which differs 

 (1) in the purple (or, to speak more accurately, deep maroon) 

 colour of some of its beds ; and (2) in containing more arenaceous 

 material. This sub-group thus consists of alternations of green 

 and purple beds. At the summit there are about 40 feet of purple 

 mudstone practically unbroken by any green bands, and showing 

 occasional thin grey flags which vary in thickness from a quarter 

 to 1| inches, and become especially prominent near the base. The 

 change in colour from green to purple is a gradual one, and the 

 purple tint fades away imperceptibly into the green. 



This baud of purple mudstone is underlain in its turn by strata 

 of a green colour having a thickness of 45 feet. Like the purple 



