Series. 



664 MISS E. M. E. WOOD OX THE [NOV. 1906, 



It is evident, from the foregoing description of this typical 

 Tarannon section, that it exhibits a collective thickness of about 

 5500 feet of strata, which may be grouped as follows : — 



Feet. 

 Wedlock |" Cb. Fynyddog Grits 1500 



Series 1 Ca " Nant -y s g ollon Sbales 450 



{_ (Zone of Cyrtograptus Murclvisoni.) 



Bd. Dolgau Mudstones. 



(Zone of Monograptus crenulatus.) 



Bd 3 . Upper Green Mudstones 1001 



Bd 2 . Middle Purple Mudstones 225 [ 420 



Bd r Lower Green Mudstones 95 J 



Tarannon ( Be. Talerddig Grits. 



(Zone of Monograptus gr lest one nsis) ... 1150 

 Bb. Gelli Sbales. 



(Zone of Monograptus crispus) 900 (?) 



Ba. Brynmair Shales. 



(Zone of Monograptus turriculatus) ... 1000 (?) 



From the top of the Fynyddog Grits to the lowest beds described 

 in the Tarannon section, there is an unbroken descending sequence 

 of strata possessing a general community of lithological characters ; 

 and the graptolitic fauna of the beds, although presenting, as one 

 descends the sequence, gradual changes due to the disappearance 

 of some forms and the coming-in of others, might, like the rock- 

 series, be regarded to some extent as a unit. 



But, on palseontological grounds, the Fynyddog Grits and the 

 underlying Nant-ysgollon Shales, which contain Cyrtograptus 

 Murchisoni, must be grouped with the Wenlock Series, of which 

 the zone of C. Murchisoni is universally regarded as the basement- 

 zone. 



The collective fauna of the rock-series composed of the four un- 

 derlying groups — which, from its grand development in this section, 

 we term the Tarannon Series — is distinct as a whole from that 

 of the Wenlock. It is divisible into four local zones, namely, 

 those of Monograptus crenulatus, M. griestonensis, M. crispus, and 

 M. turriculatus. The graptolitic faunas of these four zones are 

 quite distinct from those usually classed as Llandovery. 



(B) Confirmatory Sections. 



(1) Northern Area. 



(i) Afon Iaen and Talerddig to Llanbrynmair 

 Road-and-Kailway Section. 

 Turning now to the northern end of the Tarannon district, we 

 find an excellent confirmatory section exhibited, partly in the bed 

 of the Afon Iaen, and partly in the main road and railway from 

 Talerddig to Llanbrynmair Station which run parallel to this 

 stream. The section passes in a general north-westerly direction 

 across the northern end of the Tarannon promontory, showing well 

 the synclinal form of the country, and affording a more or less 

 continuous section from near the top of the Nant-ysgollon Shales to 

 the base of the Gelli Group. (See PL XLYIII, fig. 1.) 



