228 PEOF. S. H. EEYXOLDS OX THE LITHOLOGICAL ^vol. Ixxvii. 



of spoDge-spicules, found very little magnesia in the rocks from 

 this horizon : but as much as 1J:'5 per cent, of insoluble residue 

 was present in one case. 



(2) S.,. The Upper Seminula Beds. (Thickness ^about 

 d4l1 feet.) — These rocks form the main part of the Great Quarry, 

 and extend almost as far as the Xew Zigzag path. In the 

 association of standard limestones Avith rock-types characteristic 

 of a Jlodiola phase (calcite-mudstones and pisolites), the loNyer 

 S., Beds bear a general resemblance to those of Grower. They 

 include four well-marked diyisions (see PI. XII) : — 



S., (r^). The Seminula Pisolites and associated rocks. 

 (Thickness = about 124 feet.) — The base of S., is taken at tlie 

 outcrop of the lowest (see p. 226) band of ' Semiiuda Pisolite,' a 

 peculiar lithological type described by Vaughan (Avon paper, p. 93). 

 Sections show that the growths round the Seminidce are mainly 

 composed of Spongiosfroma (A 111). Two other pisolite-bands 

 occur respectiyely about 12 and 22 feet above the lowest band. 

 Associated with the pisolite-bands are china-stones and considerably 

 dolomitized limestones containing ILiiliosiroiion. The 'pisoliths' 

 (see PI. Till. fig. 4) haye clearly not all had the same origin. In 

 many cases the pisolith is composed of a series of concentric coats, 

 and is due solely to alsral and concretionarv growth. In other 

 cases the angular form points to brecciation, as does the fact that 

 many of the larger pisoliths haye the concenti'ic coats truncated. 

 In all the pisohtic bands the matrix betAveen the pisohths or 

 ' fragments ' is strongly dolomitized, and the appearance of 

 brecciation may sometimes be due to patchy dolomitization. 

 Silicification affects some of the Seminulce at this level. 



Above the calcite-mudstone series come massive compact lune- 

 stones of the same type as those forming the upper part of S^ and 

 like them containing veiy large masses of Lithostrofion martini 

 often silieified and associated in the uppermost beds with Dipliy- 

 ■pliyllum. Sections show these rocks to be foraminiferal, and to be 

 sometimes partly dolomitized. Semimda bands also occur. These 

 beds extend as far as the gi-assy slope in the middle of the quany 

 below the small cave. Above come variable china-stones partly 

 dolomitized, and tending to weather very white. Some of these 

 white china-stones contain A'ermiform bodies identical with those 

 already mentioned from similar rocks in S, (r/). Associated with 

 the china-stones, which may contain small gasti'opods and are 

 sometimes sti-ongly banded, is another well-marked band of 

 Semimda Pisolite ; but the deyelopment of both china-stone and 

 pisolite is patchy. 



Above the china-stone layer come about 20 feet of comjmct 

 limestone, partly dolomitized and containing in its upper part 

 three bands of chert. Some -1 or o feet of soft dolomite, suture- 

 jointed, much yeined, and tending to weather in a cavernous 

 m.anner, overlie the cherty level, and form a band which is readily 



