NODULE BEDS OF PHOSPHATE OF LIME. 103 



tabulae or septa are produced, which give a name to the concretions as 

 well as manifest their most striking character. The septa originate in 

 nests of minute concretions. If a small concretion only an inch or 

 two long is examined, the internal cracks are seen to be empty, and 

 the external surface is visibly increasing in size. We then learn 

 that, as new matter is aggregated to the outside, it is soft, and 

 consists more of clay than of carbonate of lime. Afterwards the 

 carbonate of lime becomes infiltrated into the outer clayey layer, and 

 enters into crystalline combination, so as to expand the outer layer 

 more than the internal part. This splits the concretion internally but 

 not externally, and thus, as the whole mass enlarges, the system of 

 internal cracks also becomes better developed, forming cavities in 

 which water may accumulate, and deposit various crystalline sub- 

 stances on the margins of the fractures. As already observed, septaria 

 are spread in horizontal layers, and formed out of calcareous material, 

 which was originally spread continuously through the clay, but became 

 aggregated about centres by the solvent action of waters slowly passing 

 through the deposit. 



Phosphatite. 



Phosphate of lime, in the concretionary form known as phosphatitc, 

 is perhaps more typically associated with clays, though it sometimes 

 occurs in both sands and limestones. In internal structure the masses 

 are commonly amorphous, though in some rare cases they are septarian. 

 In external appearance they differ with the several deposits. The 

 oldest known bed is found at the top of the Bala limestone, imme- 

 diately under the shale that covers that rock, and is chiefly seen near 

 the town of Llanfyllin and in the Berwyn mountains to the north of 

 Dinas Mowddwy. 1 The bed varies in thickness from ten to fifteen 

 inches, and consists of concretions which vary in size from that of an 

 egg to a cocoa-nut. They are coated with graphite, more or less polished 

 and cemented in a black matrix. They contain from 40 to 60 per cent, 

 of phosphate of lime. The underlying bed of limestone sometimes 

 contains from 15 to 20 per cent, of phosphate of lime, and when the 

 bed of concretions becomes separated into two or three layers it is 

 parted by thin phosphatic limestone. There appear to be few traces 

 of organisms in the concretions. Perhaps the most extraordinary 

 thing in connection with this deposit is the circumstance that in its 

 western outcrop in the flanks of Aran Mowddwy, sulphur has almost 

 replaced the phosphate of lime. 



Phosphoric acid is found in small quantities in some of the oolitic 

 limestones ; and the Cornbrash, so named from breaking up into a 

 soil which yields abundant crops of corn, contains a very appreciable 

 percentage of phosphate of lime. In the Speeton Cliffs, a phosphatic 

 band occurs on the horizon of the Portland beds. Another important 

 deposit is found in the Neocomian rocks of Bedfordshire and 

 Cambridgeshire. Here the deposit is concretionary, largely mixed 

 with fragments of highly altered metamorphic rocks and fragmentary 



1 D. C. Davies, Quart. Jour. Geol. Soc., vol. xxxi p. 357. 



