MOORE ABNORMAL SECOXDAEY DEPOSITS. 451 



IT. The Mexdip Hills. 



This range of hills, which gives so marked a physical character, 

 and has helped so much to modify the geology of the county of 

 Somerset, commences near Frome, and passes, almost uninter- 

 ruptedly, thence to AYeston-super-Mare on the Bristol Channel. 

 The connexion of the Mendip Hills with rocks of the same age on 

 the southern flanks of the South "Wales Coal-basin may be traced 

 by the exposui'es of Carboniferous Limestone in the islands of the 

 Steep and Flat Holmes of the Bristol Channel, and of these again 

 with small exposures of the same beds on the opposite coast. The 

 continuity of the llendip Hills with the latter is therefore pretty 

 clearly indicated ; and it is not im.probable that their elevation may 

 have been contemporaneous and due to the same physical cause. 

 Although on the South Wales coast Secondary rocks prevail, there is 

 probably but little doubt that they only lie unconformably upon the 

 Carboniferous Limestone and the Old Bed Sandstone not far beneath. 



^Measuring in a direct line from Oldford in a north-westerly direc- 

 tion to Weston-super-Mare, the length of the Mendips may be esti- 

 mated at about thirty-five miles. 



1. Old Red Sandstone. — Just north of the town of Frome, the Old 

 Bed Sandstone may be seen emerging from beneath the Inferior 

 Oolite in a narrow strip nearly a mile in length, at Spring Garden, 

 and having on its southern slope a thin band of Carboniferous Lime- 

 stone. The Old Bed Sandstone is next seen at Little Elm, whence 

 it continues in the Mendips for some miles, and is the oldest stra- 

 tified formation in this chain of hills. It has its largest develop- 

 ment north-east of the town of Shepton Mallet, and again at iS'orth 

 Hill and Blackdown ; and in the Bristol district it is found on the 

 Leigh and Weston Down. 



2. The Carlo niferoiis Limestone. — A mile west of the Carboniferous 

 Limestone previously mentioned, this formation appears in several 

 pretty combes, one leading from Frome by way of the YaUis to Elm 

 and MeUs, in the Murder Combe, and in a narrow wooded defile 

 kno"v\Ti as the Whatley Combe, in which are situated the ruins of 

 Xunney Castle. Towards the southern end of this are the quarries 

 of Holwell, the fissures in which have yielded so large a series of 

 Bhaetic organic remains. Besting upon the Old Bed Sandstone, 

 which usually forms the more elevated portion of the Mendip range, 

 the Carboniferous Limestone is seen to continue almost uninter- 

 ruptedly to the Bristol Channel, and thence along the southern por- 

 tion of the South Wales Coal-field. 



3. Basaltic Dyhe. — There is abundant evidence throughout the 

 whole of this district that the upheaval and disturbance of the beds 

 forming the Mendip Hills were caused by volcanic action ; but indi- 

 cations at the surface of the presence of intrusive rocks had never been 

 observed, probably owing to the gradual rise of their northern slope, 

 their well-cultivated surface, and the general encroachment of Secon- 

 dary deposits along their flanks. Hitherto the Old Bed Sandstone has 

 been considered the central axis of the Mendips ; but I have now the 

 satisfaction of announcing that during last summer (1866) I detected 



2i2 



