internal Structure of the Magnesian Limestone. 51 



Through the greater part of Yorkshire, north of Tickhill, the line is indicated, not by any 

 prominent feature, but by the commencement of the great plain of the new red sandstone, which 

 skirts the ridge of the magnesian limestone. In this way we may approximate to the true demar. 

 cation ; but in many places its correct determination is rendered impossible, (especially where 

 the beds of limestone are nearly horizontal,) by great accumulations of gravel and of drift sand 

 and occasionally by extensive tracts of turf land, which have been formed in consequence of the 

 imperfect drainage of so flat a district. The east side of Tickhill, Hexthorpe, the west side of 

 Doncaster, Adwick, Sutton, Askerne, the east side of Norton, Womersley, the eastern ex- 

 tremity of Knottingley, and the east side of Sherburn, are on the boundary line; and the upper 

 beds of the formation terminate abruptly in a low ridge immediately to the east of Grimstone on 

 the south bank of the Wharf. So far the general bearing of this line (with the exception of a 

 small tract north of the Don, where it is not sufficiently extended to the east) is correctly given 

 in the geological map of Yorkshire. 



For some way beyond Grimstone all traces of the demarcation are lost in the alluvial plain of 

 the Wharf: but the south bank of the river may be assumed as an approximate line as far as 

 Newton Kyme. Near that place it crosses to the north bank of the river, and has a very ill- 

 defined range ; considerably, however, to the north-east of Thorp Arch, Wetherby, and Kirk 

 Deighton. It afterwards passes immediately on the east side of Goldisbrough, and on the west 

 side of Ferensby ; crosses the valley of denudation considerably to the south-west of Staveley 

 and then sweeps round to the north-east side of Copgrove. From Copgrove the line passes on 

 the west side of Bishop Monkton, thence ranges on the east side of Hollin Close, on the west side 

 of Little Thorp, and at Ripon is lost under enormous heaps of diluvium*. 



From the last-mentioned place the demarcation, as far as it can be traced in a country so much 

 covered with incoherent matter, appears to pass considerably to the west of the road leading to 

 Tanfield ; then crosses the Ure immediately below the village of Stainley ; afterwards ranges 

 along the east side of Nosterfield and Well, skirts the east side of the road to Snape, ranges 

 about one-third of a mile west of the village, and then skirts the road to Watlas, near which 

 place, as before stated, the limestone ridge is abruptly cut offf. 



The difficulties in determining the eastern demarcation of the magnesian limestone in the county 

 of Durham are so peculiar, that they ought not to be passed over without a short notice. Here, 

 as in the other counties through which the formation passes, the back of the limestone may, on a 

 great scale, be regarded as an inclined plane dipping towards the lower region, which is occupied 

 by the new red sandstone. The accumulations of diluvial gravel are, however, so enormous, that 

 they not unfrequentlj occupy a zone several miles wide, and completely cover the boundary 

 between the two formations. In some places, also, the colour and consistency of these accumu- 

 lations make it doubtful whether we should regard them as mere heaps of diluvium, or consider 

 them as subordinate to the new red sandstone J. 



* The limestone is seen immediately to the west of Ripon ; but in the town, wells have been 

 sunk in diluvium to the depth of nearly one hundred feet. (See Plate IV. Nos. 1. and 2). 



t Nearly the whole boundary of the magnesian limestone, to the north of Copgrove, is placed 

 by Smith considerably too far to the east. 



\ The difficulty stated above appears to be acknowledged by Smith in his geological map of 

 Durham. The line of demarcation is represented as very irregular in its bearings ; and the for- 

 mation immediately superior to the limestone is described as " red and bluish clay with alluvial 

 matter." In the map published in the Society's Transactions (vol. iv. Plate I.), the range of this 

 boundary line is placed, with one or two exceptions, several miles to the south-east of any spot 

 where the magnesian limestone is visible. 



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