Part of Gloucestershire and Somersetshire. 325 



It frequently contains compressed ovoidal and spheroidal nodules and kernels 

 of chlorite, g-reen earth, calcareous spar, brown-spar, and quartz ; also balls 

 of calcedony and a^ate, the internal cavities of which are sometimes lined 

 with crystals of amethyst; and when the more perishable ingredients are 

 removed by decomposition, they leave empty cavities, whence the rock acquires 

 a vesicular and scorious aspect. Sulphate of strontian, sulphate of barytes, 

 and prehnite, appear more rarely in the trap ; which sometimes also includes 

 portions approaching to compact brown iron-stone, and brown jasper. Veins, 

 composed of carbonate of lime and brown-spar, either pure, or mixed with 

 trap and chloritic and steatitic laminae, not unfrequently traverse the rock, 

 occupying the cross fissures, which in some places divide it into cuboidal 

 and other quadrangular concretions. These concretions sometimes exhibit 

 a tendency to exfoliation, disclosing concentric lamellar layers, that surround 

 a spherical nucleus. The structure of the same mass of trap varies much 

 in the course of its extent; it is most frequently amorphous, or irregu- 

 larly divided by fissures, but when adjacent to the interstratified beds of 

 sandstone, slate-clay, and limestone, a faint tendency toward a corresponding- 

 division into strata may be partially observed ; while in some other quarters, 

 thin strata, from two to four inches thick, may be casually remarked, sin- 

 gularly contorted and inflected, yet subdivided by cross joints into rhom- 

 boidal prismatic concretions. But, in general, the internal structure of 

 the trap may be said to be independent of the interstratified disposition of 

 the beds of which it forms a part : a fact which I have had occasion to 

 notice in other instances likewise, but which does not appear to be peculiar to 

 this rock, since it is also to be observed in other formations, particularly in 

 such as are of a crystalline or of a granular texture*. But even slaty rocks 

 are not free from a similar variety of structure subordinate to their general 

 arrangement f . 



Incorporated portions of sandstone, hornstone, and limestone, both common 

 and magnesian, occasionally appear in the trap ; and in some places the rock 

 partially assumes the form of a conglomerate, a base of trap enveloping 

 rounded and angular fragments of those substances. 



A slight coating of reddish brown oxide of iron is not uncommon on the 

 separating surfaces of the stratified rocks, and particularly on those of the 

 sandstone strata, which may be traced in some cases to a thin pellicle of 



* See Geological Transactions, Vol. V. Memoir on the East of Ireland, §§ 61, 69, 71, 

 79, 172, 175, 176. 



+ Ibid. § 159. Slate-clay in limestone. — See also the Geology of Derbyshire by Mr. Farey. 

 Vol. I. Article, Limestone shale. 



