92 THE SEVERN TUNNEL SECTION. 



no doubt as to its identity, even if this eoiild not Le estab- 

 lished independently by its position. 



" From tlie distinctive cbaractcn' of tlio rocks, top^etlier 

 with the regular sequence in which they oocnr, and the 

 proximity of the outci'op of the Mountain Limestone in 

 the vicinity, we are led to believe that the Lower Goa,l 

 Measures and Millstone Grit thin out in this district to a 

 mere strip, notwithstanding that they attain greater thick- 

 ness in the Bristol Coal Field. 



" We know pr<!cisely where the bottom of the Pennant is ; 

 we have every reason to believe that 50 feet lower down we 

 are on the top of the Mountain Limestone, so that the Lower 

 Coal Measures and Millstone Grit must be wholly contained 

 in the intervening space. 



" The mixed beds of Shale, Limestone, and Sandstone 

 which occur at the bottom of Shaft No. 4, and probably also 

 for a short distance lower down, might be added to the 

 Millstone Grit, thus increasing the thickness of the series 

 to that extent ; but as thin beds of Millstone Grit are usually 

 found among the top bods of the Upper [jimestono Shales, 

 I have inoliuled this portion of the shaft in the Limestone 

 series. 



"As far as can be ascc^rtainod, tlio actua-l thickness of 

 tlie strata on the western side of the river Severn are as 

 follows : — 



Trias 85 feet. 



Upper Coal Measures unknown. 



Pennant Grit 400 feet. 



Lower Coal Measures. . . . . 35 „ 

 Millstone Grit 11 ,, 



The bed described by Mr. Jones as " close-grained red 

 firestone " is that marked "Mountain Limestone" (blue) on 

 the section. Mr. Charles liichardson informs me tha.t if 



