422 Dr. BucRLAND on Agates in Dolomitic Strata 



curves : the outermost of these curved bands are conformable to the irregu- 

 larities of the external surface, wliilst a number of minor agates nearly spheri- 

 cal in form^ and composed of the same materials as the external case^ are di- 

 spersed throughout an amorphous mass of chalcedony and hornstone, which 

 occupies the interior of the entire or mother agate. 



The prevailing colours of these bands of chalcedony and hornstone are 

 various shades of gray ; some are opaque and white, approaching to cacho- 

 long; others are red, and pass into red jasper; some of the white bands are 

 filled with minute specks of red oxide of iron, like the specks in bloodstone. 

 The central part is either a cavity lined with crystals of quartz, or a solid 

 mass of semitransparent chalcedony, or of hornstone variously coloured by 

 iron. The chalcedony is sometimes opaque and hydrophanous. 



Thus far, considered mineralogically, our specimens differ but little from 

 the common agates of the trap rocks, but their geological relations are en- 

 tirely different, and the circumstance of their matrix being a dolomitic bed 

 of the new red sandstone formation, presents a novelty worthy our attention, 

 and of which I now proceed to the details. 



It has been mentioned in the account of the south-west coal district of En- 

 gland, by Mr. Conybeare and myself, that the Mendip Hills are composed of 

 inclined strata of mountain limestone, and old red sandstone ; and that on the 

 sloping sides, and basset edges, and around the base of these inclined strata, 

 we find horizontal beds of dolomitic conglomerate, dolomite, red sandstone, 

 and red marl, which together make up our new red sandstone formation*. 



The geodes which are found in many parts of this last formation have been 

 long familiar to mineralogists under the name of potatoe-stones, and are men- 

 tioned in the memoir now alluded to (p. 292) ; these rarely contain pure chal- 

 cedony, but are mostly composed of a case or shell of hornstone or quartz, of 

 variable thickness, lined internally, and often very prettily, with crystals of 

 quartz and carbonate of lime, being almost always hollow at the centre; they 

 vary from an inch to a foot in diameter, and have evidently been produced 

 by infiltration into cavities of the matrix, in the same manner as agates are 

 infiltrated into cavities of the trap rocks : these potatoe-stones abound near 

 Wells, and also in the villages adjacent to the agate bed at Sandford, viz., at 

 Hutton, Banwell, Churchill, Winscombe, Burringdon, Compton Bishop, &c. 

 Near Cheddar and Burringdon they assume the character of coarse jasper- 

 agate : nearly pure red jasper-agates occur also in the dolomitic rocks, on the 

 left bank of the Severn, in the villages of Worle and Clevedon. 



* Geological Transactions, Second Series, vol. i. pp. 214, 225. 



