Vol. 62.] TAEANJS T OK series op taraxnox. 657 



This group of strata, which I have termed the Dolgau Mud- 

 stones, from the farmhouse of Dolgau at this locality, may be 

 referred to under the alternative title of the local zone of 

 Monograptus crenulatus, after its dominant fossil. 



When we transfer on to our present 6-inch maps the upper and 

 lower boundary -lines of the local Tarannon Shales, as laid down 

 upon the geological maps of the Tarannon district by the officers of 

 H.M. Geological Survey, at this their type-locality (see quarter- 

 sheet 60 S.W., Montgomeryshire), we find that these coincide almost 

 precisely with the top and bottom of our lithological group of the 

 Dolgau Beds. Thus, in this part of the Tarannon district at all 

 events, our Dolgau Beds and the ' Tarannon Shales ' of the Survey 

 are identical. 



(ii) Talerddig Grits (Be). (Zone of Monograptus 

 griestonensis.) 



Underneath the Dolgau Beds comes a group of strata which is 

 perhaps the most conspicuous in the district, and to its peculiar 

 lithological characteristics are due in a large measure the more 

 marked scenic features of the Tarannon heights. This group may 

 be fittingly termed the Talerddig Group, from the excellent 

 development of its strata in the railway-cutting at Talerddig, at the 

 north-eastern end of the area. 



Unlike the Dolgau Mudstones, which, as we have seen, are 

 essentially shaly, these Talerddig rocks are markedly arenaceous. 

 They consist in the main of rapid alternations of thinly-bedded grey- 

 wackes and shales, but there occur in addition numerous beds of 

 thick grit. These mass themselves together at certain horizons, and 

 form definite bands that stand out in marked contrast to the inter- 

 vening bands of thinner flags and shales. 



Thus the Talerddig Group is made up of alternations of what 

 may be termed grit- and shale-bands. The 'grit-bands' 

 contain numerous beds of grit with a variable but subordinate 

 thickness of shaly material ; the ' shale-bands ' consist in the main 

 of shales and thin flags, with only an occasional grit. 



The quartzose grits vary in thickness from 6 inches to 6 feet, 

 but only a few reach the latter size. They are for the most part 

 grey in colour, and are indeed typical greywackes. The matrix 

 varies considerably in texture, although it is usually fine-grained and 

 the included fragments are rarely even of the size of a pea ; and in 

 no case observed do they approach in coarseness to that of an ordi- 

 nary conglomerate. There is every gradation, both in texture and 

 thickness, between these more massive beds and the intervening, 

 fine-grained, micaceous flags, which may be only a fraction of an 

 inch in width. Some of the greywacke -flags exhibit cone-in-cone 

 structure, and many show the trails of annelids impressed on their 

 bedding-planes, the irregular hummocky and rippled upper surfaces 

 of which suggest that shallow-water conditions prevailed at the 

 time of their deposition. 



The interbedded shales are usually well laminated, and of a very 

 fine smooth texture. They are light to dark grey and even black,. 



