258 ' J. F. XEWSOM — CLASTIC DIKES 



of fissures by sand, which has since become indurated." On page 321 

 he notes similar dikes along Bee creek and Alderson gulch, in Shasta 

 count3\ 



Moore, 1867. In 1867 Mr Charles Moore described a number of re- 

 markable clastic dikes of Liassic age which cut Carboniferous limestones 

 in the region of the Mendip hills, Somersetshire, England.* 



The most interesting group of dikes described by Mr Moore is thta 

 exposed in a limestone quarry near the village of Holwell. A cross-sec- 

 tion of this quarry is given and the dikes shown in it are described. 



The dikes illustrated are of limestone principally and are very fossilif- 

 erous in some cases. According to Mr Moore, the limestone of the dikes 

 differs considerably in character from that of the intervening Carbonif- 

 erous stone. One of these dikes is described t as being 



" very various in color (cream-colored, yellow, pink, green, or blue), showing by its 

 occasional thinly laminated structure that it was in fact very slowly deposited. 

 This incloses angular fragments of Carboniferous limestone. In this vein may be 

 found occasional nests of Rhaetic remains mixed with Rhynchonella variabilis, Tere- 

 bratula punctata, and Delphinula nuda, Moore, and a crustacean claw of Liassic age. 

 At the base of the quarry it attains a thickness of 13 feet." 



Mr Moore calls attention to the fact that a number of similar dikes 

 other than those described occur in the Holwell locality. That the Li- 

 assic dikes are not confined here to one small locality is show^n by the 

 fact that 9 miles west of the Holwell locality a dike occurs in a large 

 quarry. 



Eight miles still farther west, at the Charter House Liassic lead mine, 

 2 J miles distant from the nearest known horizontal deposit of Liassic age, 

 Mr Moore found in an old shaft, at a depth of 270 feet, '' a deposit of deep 

 blue or greenish clay, 12 feet thick, giving the appearance of having been 

 deposited in thin horizontal layers, therefore slowly, whilst at other spots 

 it presented a more conglomerate character, and contained driftwood 

 pebbles, etc.," and from which were collected about 95 species of ver}^ 

 delicately preserved lower Liassic fossils. 



As indicating the mode of formation of this deposit. Mr Moore says : 



*' There can be little doubt that the Liassic seas at this period occupied the pro- 

 found depths of the Carboniferous limestone fissures, within which the organic 

 remains were probably living, contemporaneously with the deposition of Liassic 

 beds at other points. The delicacy and perfect condition of the fossils show that 

 their presence is not due to the denudation and the redisposition of previously 

 existing beds within the fissures." 



* Charles Moore : Oa abnormal conditions of secondary deposits when connected with the Somer- 

 setshire and South Wales coal basin, and on the age of the Sutton and Southerdown series. Quart. 

 Jour, of the Geol. Soc. of London, vol. xxiii, 1867, pp. 48) et seq. 



fLoc. cit., p. 485. 



