348 Mr. Weaver's Geological Observations on 



parts of the kingdom. Their general position and extent, in relation to the 

 tract under consideration, as well as those of the succeeding lias and oolitic 

 limestone, have been already described in § 4. and § 5., and in part also illus- 

 trated by the map and sections 1. 2. and 3, Plate XXXIX. 



§ 26. The characters of the calcareous conglomerate have been so fully 

 given by Mr. Horner (Geol. Trans. Vol. III.)*, and by Dr. Bright, Mr. 

 Warburton, and Dr. Gilby (Geol. Trans. Vol. IV.) ; that a short description 

 of the formation, as appearing in this district, will suffice. It is composed 

 principally of rounded and angular fragments of limestone and sandstone, 

 sometimes exceeding the size of the head, with fragments also of hornstone 

 and quartz : these are cemented by a calcareous paste, which is frequently of 

 a marly nature, or a common carbonate of lime either of an earthy or com- 

 pact structure ; but in some quarters, as in the vale of Thornbury, the cement 

 is generally magnesian, and through all the varieties, sandy particles are more 

 or less distributed. The compound abounds in cavities which are frequently 

 lined with crystals of calcareous spar and quartz, and sometimes also with 

 sulphate of strontian. The formation^ where it is in contact with older rocks, 

 is generally disposed conformably to the declivity presented by their surface ; 

 but the higher strata soon acquire an horizontal arrangement. It thus extends 

 from the western escarpment of Milbury heath into the vale of Thornbury ; 

 and in the latter quarter, it is well exposed in a small precipice on the south- 

 western side of Thornbury castle and church, which edifices are founded on 

 the conglomerate. The upper part of the rock is rubbly, the lower part per- 

 fectly consolidated, in nearly horizontal strata, about four feet thick. In the 

 vale of Thornbury, this conglomerate in some portions passes into magnesian 

 limestone, the general character of which is that of a fine and minute foliated 

 granular rock, of various shades of gray, yellow^ and red. In the limestone- 

 fragments of the conglomerate, entrochites, terebratulites, and productites, 

 sometimes appear ; those fragments being evidently derived from the car- 

 boniferous limestone. To the east of Wickwar, on the other hand, the 

 conglomerate supports a yellow, earthy, magnesian limestone, the upper 

 part of which alternates with the red clay-marl of the new red sandstone for- 

 mation, in layers two or three inches thick. 



In the conglomerate on the northern brow of Tortworth hill, besides the 



* In the part of England, which is considered in Mr. Horner's valuable paper, the magnesian 

 limestone foimation seems to be wholly wanting ; and hence the calcareous conglomerate of that 

 tract is not unfrequently in contact with the new red sandstone formation, forming its immediate 

 support. 



