148 c. L. :sioega:n- ox the s.w. extension 



.Mr. Stoddart's estimate (Geol. Mag. vol. ii. p. 83) of the thickness 

 of the Upper Limestone Shales at 600 feet, which I think is a fair 

 one, there will be 160 feet or so of these beds above Ligh-water 

 mark, above which the Millstone Grit will be brought down. This 

 accords very well with the facts, that contorted grits and shales are 

 seen from the new road some 50 or 60 feet above high- water mark, 

 and that above this little but grit is to bo found. It must be 

 remembered, however, that the beds in the immediate vicinity of 

 the fault on the down-throw side are exceedingly contorted. 



A curious effect of the general squeeze by which the rocks here 

 are literally " in one red burial blent," is well shown in the immediate 

 neighbourhood of the line of fault on the Gloucestershire side of the 

 river. Here, over a face of limestone dipping 18° K"., has been thrust 

 a semicircular mass of rock with an external layer of grit, then a 

 band of limestone, and internally a core of grit. 



Further eastward on this same Gloucestershire, or Chfton, side of 

 the Avon, the effects of the fault on the physical features of the 

 country are well marked, causing the well-defined depression in 

 which lie Triassic beds between the Clifton Downs (with its tongue- 

 shaped mass of limestone stretching westward from the Suspension 

 Bridge) on the south, and the northward-trending limestone mass 

 of Durdham Downs on the north. (See map.) 



Passing now to the opposite, or Somersetshire side, we find the 

 following features, as seen from such a standpoint as the Observatory 

 Hill. A little to the north of the Suspension Bridge is the deep 

 indentation of Mghtingale Yalley, the southern side of which is here 

 precipitous, and answers to the vertical face which forms the 

 southern boundary of the station-recess on the Grloucestershire side 

 of the river. The northern side of the valley is less precipitous, 

 forming, in fact, more or less of a dip slope of limestone. A hundred 

 and fifty yards or so further north is another precipitous face of 

 limestone, that caused by the fault. After this, as we pass north- 

 wards, there is, on this side of the river as on the Clifton side, a 

 wooded slope. The solid limestone rises from the river-side at a 

 distance of some 2-10 paces from the faulted limestone face. 



Owing to the fact that the fault cuts across the strata obliquely, 

 lower beds of the Mountain Limestone are intersected on this side 

 of the Avon than were cut on the Clifton side. Hence the rocks 

 relativelj' shifted downwards at the point of intersection of the line of 

 fault and the line of high water here are Mountain Limestone, about 

 780 feet, and Upper Limestone Shales about 330 feet, making a rela- 

 tive displacement of 1100 feet. Eemembering that the Upper Lime- 

 stone Shales are some 600 feet in total thickness, this leaves 270 feet 

 of these rocks, which would thus just extend to the top of the cliff. 

 Under these circumstances we should not expect to find Millstone 

 Grit faulted down on this side of the river. jN'or have I on careful 

 search been able to discover any beds of this rock. At the point 

 marked h on the map, however, I have found well-defined bands of 

 grit interstratified with limestone, showing that we here have beds of 

 the Upper Limestone Shale. 



