'660 MISS E. M. E. WOOD ON THE [Nov. I906, 



Some 120 feet below this graptolitic locality, a single grit-bed 

 14 inches thick is exposed, and in the intervening shaly strata similar 

 well-preserved graptolites were found. This 14-inch grit is almost 

 the last seen in the Tarannon section, and it may conveniently be 

 regarded as forming the base of the Talerddig Grits. 



Reviewing the Talerddig Group as a whole, we see that it 

 consists of alternations of grits, greywackes, and shales, having 

 perhaps a collective thickness of 1100 to 1150 feet. The arenaceous 

 beds are, as a rule, only a few inches thick ; but at five distinct 

 horizons they increase to between 1 and 4 feet in thickness, and are 

 more or less massed together to form compound grit-bands. 



The fauna of the Talerddig Group is similar throughout, and while 

 it is distinct as a whole it is intimately related to that of the Dolgau 

 Beds above. Eight species have been detected, of which all but one 

 (M. Marri) range up into the overlying group. Monograptus crenu- 

 latus, however, which is abundant in the Dolgau Beds, makes its 

 appearance only in the highest fossiliferous seam of the Talerddig 

 Group ; while M. griestonensis , M. discus, M. nuclus, and M. sub- 

 conicus, which are doubtfully or poorly represented in the Dolgau 

 Shale, are at their maximum development in the underlying 

 Talerddig Grits. The most characteristic and abundant species is 

 undoubtedly M. griestonensis, and the Talerddig Group may there- 

 fore be referred to under the alternative title of the Monograjptus- 

 griestonensis Beds. 



As we have already seen, the officers of H.M. Geological Survey 

 regarded the strata immediately underlying their Tarannon (Dolgau) 

 Shale of this special section as of Lower Llandovery age, and 

 distinguished on the map two bands — an upper shaly and a lower 

 gritty one. Their Lower Llandovery Beds include the upper half 

 of our Talerddig Group : their upper shale-division answering to our 

 6th shale-band, and their lower grit-division to the beds between the 

 5th and 3rd grit-bands. Both on lithological and paheontological 

 grounds, however, the base of the Talerddig Group should be carried 

 down to the lower horizon which we have chosen. 



So long as the beds referred to were regarded as of Lower Llan- 

 dovery age, a stratigraphical break presumably existed between the 

 Talerddig and Dolgau (Tarannon) Beds. But all the evidence — 

 lithological, stratigraphical, and palseontological — now obtained goes 

 to show that there is a complete gradation between the two. 



(iii) Gelli Shales (Bb). (Zone of Monograptus crispus.) 



The Talerddig Beds pass down with complete conformity into 

 a thick group of strata, which lithologically differ from them mainly 

 in the absence of the grit-bands. They consist of alternations of 

 well-laminated, light-grey, blue, and black shales with occasional 

 mudstones and small micaceous flags (a quarter to 2 inches thick), 

 the latter decreasing in number and thickness as one descends the 

 section. 



