ON EXCAVATIONS IN TttE PALEOZOIC ROCKS OP WALES, ETC. H3 



Qiitly upon the Lower Coniley Sandstone, and that the discordance is 

 not due to faulting is rendered highly probable by the existence of 

 cavities or burrows in the lower strata, which are filled with the material 

 of the upper beds. 



At two fresh spots along the line of the new excavation the same 

 surface of discordance was laid bare. They are shown in fig. 1 and 

 marked Xi, Xa. In these, as in that of excavation 29a, the surface of 

 discordance rises towards the south-west. 



At 29d the eroded surface takes a sinuous line, which is roughly 

 horizontal and a distinct hollow is shown, but the bedding of the 

 superior group is obscured, within the limits of the excavation, by sur- 

 face debris. 



It will be seen from the section, fig. 1, that the Quarry Ridge 

 Grits occur in patches on the surface of Robin's Tump above the excava- 

 tion, and may perhaps there form a practically continuous outcrop above 

 the Lower Cambrian. The four repeats of the basement bed shown in 

 the figure along the line of the new excavation are, in my opinion, due 

 to little faults, some of which are actually visible in the excavation 

 itself. 



The right (south-west) portion of the section is the excavation 29a 1 

 of the Sheffield Report. In this a rib of dark calcareous rock protrudes 

 from the eroded surface of the Lower Comley Sandstone. This rib 

 allows of a pocket to the right of it, the sides and base of the pocket 

 being all visible. This pocket contained the fossiliferous rock blocks 

 alluded to in the Third Report as 'nodules.' Further study and the 

 new excavations made it clear that these blocks, which contain Lower 

 Comley fossils, are actually pebbles lying in the pocket of erosion and 

 in a matrix of Upper Comley (Quarry Ridge Grit). From the gritty 

 matrix surrounding these pebbles I collected a free cheek of Parad- 

 oxides, only the pebbles themselves afforded me the Lower Comley 

 fossils. 



In a transverse cleft in the projecting calcareous rib two or three of 

 these Lower Comley limestone pebbles are to be seen, wedged fast with 

 the gritty matrix of the Paradoxides beds. Of this exposure a good 

 photograph (Plate III., fig. 3) was secured, showing the calcareous rib, 

 ABC; the pocket, D; and the pebbles in the cleft, under the letter B. 

 The large block above belongs to the upper series, but is not in situ. 

 The new excavation also showed two additional ribs of the calcareous 

 rock as integral parts of the Lower Comley Sandstone, but their con- 

 tact with the superior beds is not visible. These are indicated by the 

 letters Y,, Y 2 on the section, and from its position and thickness I am 

 of opinion that Y x is the same band as that encountered in excavation 

 No. 29c (Sheffield Report), where the contact was observed. Y 2 is a 

 much thicker band, and probably not a repetition by faulting of either 

 of the other two bands. 



Visible Discordance of Dip and Strike between the Lower and Upper 



Series at Robin's Tump. 



As pointed out in the Third Report, the beds referred to as Lower 

 Comley appeared in excavation 29a to show a discordance both in dip 

 1911. I 



