10 THE GEOLOGY OF TYTHERINGTON AND GROVESEND. 



Bay. I Lave not yet proved it in tlie Mendips ; but here we 

 have it on the northern borders of the Avon Basin near 

 Grovesend. 



It is difficult here, as elsewhere, to determine where 

 the so-called Lower Limestone Shales pass into the Old Eed 

 Sandstone. The series is essentially a transition series. 

 Above, it shades into the Lower Limestones of the Car- 

 boniferous : below, it shades into the Old Red Sandstone. 

 Above, there are shales with limestones ; below, there are 

 shales with sandstones. I have carefully gone over the beds, 

 testing them with acid, and am disposed to place the in- 

 coming of the Old Eed Sandstone at a point 480 feet from 

 the bridge. Here occurs a band of hard, close-grained sand- 

 stone. Below it, none of the beds show any effervescence 

 with acid ; shortly above it, they give the characteristic 

 calcareous reaction. The bed I speak of is 120 feet from 

 w^here the conglomerate with milky quartz, shown on Mr. 

 Meredith's section, crosses the line. I estimate the thick- 

 ness of the Lower Limestone Shales between the points I 

 have indicated at 315 feet. 



The succeeding beds to the tunnel mouth consist of red 

 shales, sandstones, and conglomerates, with milky quartz 

 abundant. Just at the tunnel mouth are hard conglomeratic 

 beds ; and a similar bed is seen, as before mentioned, above 

 the road beneath which the tunnel passes. 



Subsection 5. — On emerging from the tunnel coarse con- 

 glomeratic and brecciated Basement Beds of the Trias are 

 seen resting on the vertical edges of upturned Old Red con- 

 glomerates and shales (see Eig. 2). Of the Basement Beds I 

 shall speak presently. Here I need only note that there are 

 two other places where the Old Red is exposed at the base 

 of the section. The first of these begins 265 feet from the 

 tunnel mouth, and lasts 110 feet. It shows the summit of 



