MOOEB ABNOEMAL SECONDARY DEPOSITS. 



455 



Rliaetic, Liassic, or Oolitic age. Parallel veins, analogous to the mineral 

 veins of other districts, traverse the Mendips in a direction east and 

 west, from one end to the other, and are occasionally intersected by 

 others from north to south. They appear to pass downwards through 

 the whole thickness of the Carboniferous Limestone, and have been 

 proved at Charter House to a depth of 260 feet. In general they 

 are of Liassic age ; but the mineralogical and palaeontological variety 

 they present show that they were not formed contemporaneously. 

 Probably they were for a long time open to the Liassic seas, and 

 must in many instances have received their contents very gradually ; 

 a Liassic fauna not only inhabited the ocean above, but lived within 

 the Carboniferous Limestone walls of the open fissures, and the 

 remains of Gasteropoda and other organisms may still be seen at- 

 tached thereto. As we are able to arrive at the age of horizontal 

 strata by the fauna they enclose, so are we able to indicate in 

 many cases the probable relative age of mineral and other veins by 

 the same palaeontological law. 



So numerous are these fissures in the Mendips, and so much of 

 what appears Carboniferous Limestone is really of Liassic date, that 

 the geological character can only be expressed on the Ordnance Map 

 by drawing parallel lines of Lias throughout their length. 



7. Age of the Conglomerates. — In the south-west of England and in 

 "Wales the Permian series, which in the north is found resting upon 

 the Coal-measures, appears to be entirely wanting ; and within the 

 centre of the southern division of the basin, conglomerates, probably 

 of Dolomitic age, lie unconformably upon the latter. These are found 

 to vary from 3 to 60 feet in thickness. There is a large develop- 

 ment of this conglomerate at Stratton-on-the-Posse, where it comes 

 in contact with the exposed edges of the Coal-measures, and 

 again at Mells, whence a narrow belt passes beyond the village of 

 Elm, skirting the limestones to YaUis. Conglomerates are also 

 found continuously along the limestone escarpments, filling up the 

 inequalities of the surface on both the north and south flanks of 

 the southern portion of the Mendips. It is very difficult to obtain 

 sections showing the age and relative stratified positions of many of 

 these conglomerates ; and few, if any, organic remains of a locality 

 so disturbed as were the probably local coast-lines on which these 

 pebbly deposits were laid down can be expected to have been pre- 

 served. Although it may be difficult of absolute proof, I have reason 

 to believe that these conglomerates were accumulated at difi'erent 

 geological periods. Those which rest unconformably upon the Coal- 

 measui'es in the basin north of the Mendips are undoubtedly the 

 oldest in the district, and probably represent the true Dolomitic 

 period, as do many of the patches along this chain of hills. But the 

 Bhaetic period is not only represented by horizontal conglomerates, 

 but also in the vertical fissures of the limestones, by material which 

 would, under ordinary conditions and without the presence of organic 

 remains, pass for unquestionable Dolomitic conglomerates. The Car- 

 boniferous Limestone of Durdham Down, near Bristol, is traversed 

 by numerous fissures similar to those already alluded to as occurring 



