DEPOSITS OF NORTH WALES. 359 



that limestone is overlain (see section, fig. 1). The bed varies 

 in thickness from 10 to 15 inches. It is black in colour from the 

 graphite it contains ; and its appearance is that of a number of con- 

 cretions, which range in size from that of an egg to that of a full- 

 sized cocoa-nut, closely packed and even running into each other, 

 and cemented together by a black matrix. The concretions have 

 often a polished appearance, which is also due to the presence of 

 graphite; and frequently along the course of the bed the phosphorite 

 is charged with concretions and crystals of sulphide of iron. Near 

 the outcrop of the bed this becomes oxidized, and the deposit changes 

 its black for a rusty appearance. The concretions, as will be seen 

 by the analyses which accompany this paper, contain the most 

 phosphate of lime, usually 64 per cent.; but the matrix also contains 

 a portion ; so that the average yield of the bed is about 46 per cent. 

 Was it from the disintegration of a somewhat similar bed that the 

 rolled and more widely diffused phosphatic nodules of Bedford, 

 Cambridge, and Suffolk were derived ? 



The bed is underlain by a thin bed of crystalline limestone, which 

 does not usually exceed 6 inches in thickness, though there are a 

 few exceptions to this rule. This also contains phosphate of lime, 

 sometimes to the extent of 15 or 20 per cent. There is but 

 one bed of phosphorite ; and it is very persistent in its continuity, 

 and extends over a large area. Indeed there cannot be any doubt 

 that wherever the Bala Limestone is found in North "Wales or 

 the borders, this deposit will be found, more or less pure, at its usual 

 horizon. 



Sometimes, however, the bed divides, mostly into two, but some- 

 times, as in the Berwyn Mine, into three beds. When this division 

 takes place, the dividing substance is the thin phosphatic limestone. 

 The uppermost bed at such times dies out as it enters the overlying 

 shales. So also does the middle one. It is invariably the lowest bed 

 which is continued forward, the overlying limestone dying out until 

 the shales take their true position immediately above the phosphorite 

 bed. 



-It may help to the better understanding of the whole subject if 

 I now describe generally the strata with which the phosphorite bed 

 is associated. It lies, as I have said, at the summit of the Bala Lime- 

 stone. At Blaen Rhiwarth, where the Berwyn Mine is situated, and 

 at a depth of about 300 feet below the phosphorite, the limestone is 

 seen gradually passing out of the ash-bed (which close by becomes a 

 compact greenstone) and assuming a calcareous character. Its lower 

 beds are massive and are charged here and there with their charac- 

 teristic fossils. Higher up the beds become thinner and are inter- 

 stratified with shales of various textures, until at last the whole deposit 

 is capped by the phosphorite, and shales permanently follow. 



To describe the fossils of the beds below the phosphorite would 

 be to enumerate the whole of the fossils of the Bala Limestone. I 

 would therefore now only direct attention to two or three features 

 in the grouping of these, which are chiefly remarkable along with 

 the phosphorite bed by their continuity over a large area. 



