402 Horace B. Woockvard — On the Metamorphism of Strata. 



these beds of chert, according to Weaver, comes the Conglomeratic 

 bed, which is probably of Eheetic age. 



Near the cottage at Harptree Hill is a pit dug in hardened reddish- 

 brown sand ("rock-sand") crowded with Pullastra arenicola, while 

 in hollows hard by loose blocks of sandstone containing this shell, 

 together with Avicula contorta and Fecten Valoniensis, are to be 

 found. These Eheetic sands show no other alteration from their 

 original character than induration. They are of very local distri- 

 bution, and occupy a position above the grey Eh^tic marls ; they are 

 not exposed along the escarpment to the north, but here the ground 

 is much obscured by detritus. The lowest beds of the Eh^tic, the 

 grey marls, pass downwards into the -red marls of the Keuper at the 

 East Harptree Lead Mines, and show no alteration in character. 



Around Green Down Cottage the ground is very complicated, 

 and here there is a quarry exposing the ordinary argillaceous lime- 

 stones and clays of the Lower Lias and a portion of the White Lias. 



South of the cross roads at Egar Hill are beds of Lias Limestone. 



These facts show the partial nature of the silicification, and also as 

 regards the Lower Lias and the White Lias, that there was no 

 gradual change in sediment from limestone and clays to an are- 

 naceous beach deposit, which is an explanation that suggests itself. 



Keuper Beds. — The Chert beds associated with the Keuper occupy 

 very variable positions ; they can be traced sometimes above, at others 

 below, the Eed Marl or Dolomitic Conglomerate, which latter occa- 

 sionally appears silicified. On the borders of the Mendip Hills it is 

 well known how these beds dovetail one into the other. The ques- 

 tion is, whether these beds be altered red marls, or whether they 

 were originally arenaceous, and have been subsequently compacted. 

 I am inclined to think the former supposition the correct one, 

 because in this immediate neighbourhood, where sections of un- 

 altered beds are obtained, as in the lane leading from Compton 

 Martin Church up Keighton Hill, and also in the gorge between 

 Noah's Ark and West Harptree, marls and conglomerates alone occur. 

 In the quarries at East Harptree, where the beds are worked for 

 road-metal, they appear much broken up, and there are no lines of 

 stratification apparent. These beds are described by Messrs. Buck- 

 land and Conybeare as sandstones "of so fine and compact a grain 

 as to assume the character of chert." They have a peculiar baked 

 appearance, and are quite unlike the ordinary sandstones of the 

 Keuper series, the only representatives of which in the neighbour- 

 hood are some calcareous sandstones or cornstones found near Chew 

 Magna, etc., and which are quarried for building purposes. 



Character of Change. — It is clear, then, that the Cherty beds have 

 sufi'ered a great change since their original deposition, and that only 

 local patches of rock have been metamorphosed. What now was 

 the agency that produced this change ? We look in vain for any 

 suggestion in the district itself, and therefore seek information else- 

 where. Turning to Jukes's Manual,^ we learn that the Lias of Port- 

 rush has been converted by contact with a large mass of greenstone 

 1 New (second) edition, 1862, p. 166. 



