THE SEVEEN TUNNEL SECTION. 91 



met with, and tlioir perfect geological sequence. Tins is 

 sliowu in. a striking manner in Shaft Wo. 4. This shaft is 

 only 200 feet deep ; nevertheless, in sinking through, that 

 distance, the New Red Sandstone, Dolomitic Conglomerate, 

 Pennant Grit, Lower Coal Measures, and the Millstone Grit 

 occurred within a vortical depth of 154 feet. 



" The vertical thickness of the Pennant Grit here is 19 

 feet, as stated, its dip averaging 4-75°, and its horizontal 

 length 1,000 yards. Calculating from these data, its actual 

 thickness may be stated as 400 feet, or less than a fourth of 

 its average thickness in the Bristol Coal Field; hut the 

 greatest anomaly is in the thickness of the Lower Coal 

 Measures and the Millstone Grit, which here have an actual 

 thickness of only 35 feet and 11 feet respectively, as against 

 2,000 and 1,000 feet in the Bristol Coal Mold. 



" In the cutting near Portskewet station on the South 

 Wales Railway, nearly half a mile to the north-west of the 

 shaft, the Mountain Limestone is exposed, coming very 

 ii(!:u'ly to the surface, dipping E.S.13., or nearly in the 

 direction of the shaft, at an angle of about 5°.* From tiiis 

 it would appear that the Limestone ought to bo found at the 

 shaft at a depth of about 200 feet from the surface. Very 

 nearly at this depth the peculiar lumps of Limestone in 

 Shale, and the interlaminated beds of Limestone and Shale 

 were found, which show that in all probability we are there 

 on the top of the Mountain Limestone. 



" It may be mentioned with regard to the bed of Millstone 

 Grit, that the distinctive character of the rock would leave 



* The average clip of tlie Carboniferous bods, ascertained by ob- 

 servations taken in tlie shafts and headings, and by comparisons of 

 the relative po.sitions of tlie bods in the shaft with those in the headings, 

 is about 475° corresponding to a fall of 1 in 12. 



