Vol. 62.] ROCKS OF WESTERX CAERMARTHEXSHIRE. 613- 



Ant pyx sp. 



Tri nucleus cf. Salter i, Hicks. 

 Mylina binodosa, Salter (large;. 

 irca sp. ; etc. 



Didymograptus extensus, Hall. 

 Didymograptus pat id us, Hall. 

 Tetragraptus Amii, Lapw. 

 Tctragraptus serra, Broiign. 



Still higher up, at Tynewydd, some mudstones dug out of a well 

 were crowded with Didymograptus extensus, Hall. Associated with 

 these were Barrandea sp. ; Trinucleus cf. Murchisoni, Salter ; 

 Hlcenus (?) ; Lingula sp. ; Palcearea, etc. Didymograptus-bifidus 

 Beds are exposed a few yards farther north ; and it is almost a 

 certainty that the junction is a faulted one, and that a portion of 

 both series has been cut out here. 



(6) Conclusions. — Reviewing the stratigraphical and fossil 

 evidence, the conclusion is reached that the Arenig 

 rocks of this district are the equivalents of the 

 Tetragrajptus^-Beds of St. David's and are of Middle 

 and Upper Arenig age. There is not sufficient evidence to 

 prove that the Lower Arenig rocks are represented in Western 

 Caermarthenshire. 



The Middle Arenig strata (the equivalent of Hicks's ' Lower 

 Arenig ') here naturally divide themselves into two sub-groups — 

 (a) the Blaencediw Dictyogr aortas-Beds with their associated grits, 

 and the Henllan Trilobite-Beds ; and (/3) the higher shales and 

 mudstones with Tetragraptus and Didymograptus extensus, etc. 



The Upper Arenig of this district includes all other beds with 

 Didymograptus hirundo, and other extensiform graptolites, up to and 

 including beds with Diplograptus and Climacograptus without 

 Didymograptus Lifidus. 



The complexity of the ground is such, that no attempt has been 

 made to draw definite lines separating the Upper from the Lower 

 Arenig rocks of the district. 



(b) Llanvirn Group : Didymograptus-bifidus Beds. 



The D.-bifidus Beds of the district consist of a series of blue-grey 

 and blue-black mudstones and shales, thin grits, and bands of ash. 

 The mudstones and shales are often stained light chocolate. 

 They weather in irregular patches of light buff and fawn, and when 

 viewed edgewise they present an irregular streaky appearance. 

 Where they are cleaved they assume a peculiar sheeny lustre, 

 probably due to shearing. The grits are blue-grey in colour and 

 much jointed. The ashes, freshly fractured, are light blue, but on 

 weathering they present a buff or cream-coloured groundmass with 

 specks of rusty material, formed probably by the decomposition of 

 pyrites and other minerals containing iron. Some of the weathered 

 beds are quite vesicular, owing to the decomposition of minerals con- 

 taining lime. Calcareous pea-like masses are found in half- weathered 

 samples. 



The rocks have been subject to a great deal of folding and 

 faulting, and the beds are frequently repeated, the ash-bands 

 suffering with the rest. There are, however, at least two distinct 



