270 VOLCANIC GRITS ALTERNATE WITH LOWER SILURIAN STRATA. 



canic grits and breccias, their conchoidal fracture when first extracted being apparent in the loose 

 blocks of the foreground 1 . 



Passing from Marrington Dingle to the east or south-east, we meet with the following alterna- 

 tions of rock : 



4th. Flags and shale. 5th. Stratified trap, breccia and grit, of which more will be said in describing other parts of the 

 district. 6th. Various alternations of shale and trap-tuf, which extend to one of the longitudinal furrows or denudations, the 

 surface of which is covered by yellow clay. 



All these masses, except the last, are conformably inter-stratified, and overlie a system of black 

 flags, containing at Marrington and Middleton the Asaphus Buchii and other trilobites. A pa- 

 rallel zone of trap runs from the Lower House by Kinton to Wilmington, a distance of more than 

 two miles, and although clearly of intrusive character it may be here described to complete this 

 section. Between Rorington and Wotherton a quarry in this band shows it to be there thirty feet 

 wide. The upper and lower surfaces of the trap are precisely parallel to the beds of schist and sand- 

 stone dipping 60° north-north- west ; the trap itself not being divided into beds, but arranged in 

 masses more or less prismatic. It is an amygdaloidal greenstone in a decomposing state, the 

 kernels, about the size of a bean, consisting of calcareous spar and a few crystals of iron pyrites 

 disseminated through the mass. On following its strike to the north-north-east this band reappears 

 at Wilmington, where it has about the same thickness. Here, however, the mass is not decomposed, 

 and contains kernels of crystallized olivine and calcareous spar. In this, as in the former case, the 

 sides are parallel to the inclined strata, the trap arranging itself into four and five-sided columnar 

 forms, the ends of the prims being at right angles to the walls of sandy schist as expressed in the 

 wood-cut 44, p. 275. At a few paces distant from and overlying the dyke I found specimens of 

 Asaphi, the beds containing them dipping to the west-north- west. 



Transverse sections to the flanks of StapelyHill, by Middleton, Rorington, or Meadowtown, ex- 

 hibit the black flags, with Asaphus Buchii and other species, rising from beneath all this system of 

 shale, sandstone, and trap, in the following order : 



8th. Black shale and true Llandeilo flags loaded with both the species of trilobite, viz. Asaphus Buchii and A. tyr annus, 

 so common in Caermarthenshire. 9th. Stratified felspar rock, almost compact. 10th. Volcanic grit and sandstone of dingy 

 green colour, passing downwards into felspar flags, with many fragments of trilobites. 11th. The same greenish grey flags, 

 but of a more crystalline aspect, composed of grains of felspar, quartz, and hornblende. These also abound in casts of tri- 

 lobites, the surfaces of the beds being also marked by branching bodies, fucoids? 12th. Black shale and sandstone. 



In the whole of this series, as exhibited on the sides of a ravine, the strata are perfectly conform- 



40. 



From a drawing of Mr. T. Webster, F. G.S. 



1 This stone is now coming extensively into use for building and is of excellent quality. 



