i£OOEE ABXORiTAL SECOXDAHT DEPOSITS. 453 



Passing to the Trestern side of the dyke from Stoke Lane, the Old 

 Eed Sandstone again forms the most elevated portions of the 3Iendip 

 Hills by way of Pen Hill, Xorth Hill, near Priddy, and Blackdown — 

 in which direction, as a general rule, all the older rocks have a north 

 and south dip, which would necessaiily be the case throughout that 

 range if their upHfting had been the result of any volcanic force 

 exerted from beneath the Old Eed Sandstone. 



A north and south section across the Xettlebridge valley shows 

 that, by the protrusion of the dyke, rocks of enormous thickness, com- 

 prising those fi'om the Old Red Sandstone to the Coal-measures, have 

 been carried bodily forward in a northerly direction for a great dis- 

 tance, and are not only left standing vertically, but are in some in- 

 stances folded over upon themselves. In consequence of this there 

 is the extraordinary physical phenomenon of a band of Carboniferous 

 Limestone, one-third of a mile in length, at Luckington, which appears 

 to have been carried away from the llendip Hills, a mile and a half 

 to the south, under which reversed Coal-measures are being worked ; 

 and that which formed the floor of the bed is now found to be its 

 roof. 



4. Date of Upheaval. — It will be important, as far as possible, to 

 determine the relative time when the mighty agent to which I 

 have aUuded produced such physical changes in the districts under 

 notice. This is to be arrived at in the first place by an examination 

 of the rocks that have been subjected to its elevatory force, and next 

 of those which have been subsequently deposited. A section of the 

 Mendips fi'om north to south at once shows that the only rocks which 

 have been operated upon in this way are the Old Eed Sandstone, the 

 Carboniferous Limestone, and the Coal-measures. These have pro- 

 bably an aggregate thickness of 20,000 feet. Although there are 

 local disturbances in deposits more recent than the Coal-measures, 

 they are due in great part to subsequent oscillations ; and it may be 

 seen that the Secondary deposits, although presenting remarkable 

 evidences of unconformability when they reach the flanks of the 

 Mendips and come in contact with their older uplifted rocks, are in 

 general horizontal and undisturbed. Many of the Secondary rocks 

 will be shown to be absent ivifhin the Coal-basin, or, if present, to 

 be very feebly represented : and we thus arrive at the conclusion that 

 the elevation of the Mendips and their South "^^ales continuation 

 may be assigned to a time not far removed from the deposition of 

 the upper beds of the Trias or Xew Eed Sandstone. 



Having arrived at an approximate date respecting the upheaval of 

 the gTeat mass of the Mendip chain, it will be desii-able to ascertain 

 what evidence the later Secondary rocks afi'ord on this head, and how 

 they have been modified thereby. From a consideration of this 

 point, I am unable to agree with that eminent physical geologist, 

 Professor Eamsay, in his opinion that rocks to the thickness of a 

 mile have been denuded from the summit of the Mendips, and from 

 the districts surrounding the South Wales Coal-basin. In this case 

 we should expect to find the denuded rocks entii^ly absent from 

 the district ; but it can be shown that manv of the Secondary rocks 



