Vol. 60.] LIMESTONE AT BARNWELL. 361 



of which at least three types occur : these we may distinguish as 

 the greeu, the pale, and the dark-brown. 



The green nodules are more or less irregular, subcylindrical 

 lumps of phosphatized mud. They enclose no obvious shell-frag- 

 ments, and never contain more than about 10 per ceut. of calcium- 

 phosphate. They seem to represent mud-pebbles deposited among 

 the shells, and have probably become phosphatized in situ. 



The pale nodules (which are sometimes yellow, sometimes 

 brown) are very similar in structure to the green, but contain a 

 much greater proportion of calcium-phosphate, generally about 35 to 

 40 per cent. They are very irregular in shape, but are never obviously 

 rolled. A few seem to be the internal casts of shells now destroyed. 



The dark-brown nodules are less common; they differ from 

 the others in that the}* have well-rounded shapes, and appear to have 

 been derived from older beds. They are much richer in phosphates 

 than an)' of the others, analysis showing something more than 

 50 per cent, of calcium-phosphate. They are often bordered with 

 material like that of the pale nodules, and contain no determinable 

 fossils. 



Under the microscope, the rock l is seen to be made up of more or 

 less recognizable shell-fragments. About one-half of it consists of 

 the fibrous calcite-prisms characteristic of Inoceramus. Uorami- 

 nifera are also very abundant, and many forms occur. Globk/erina, 

 Jlillola, Nodosa ri a, and Textilaria are the most prominent genera. 

 As usual, they have the chambers filled with calcite, which is in 

 crystalline continuit} 7 with the test, and so shows the usual black 

 cross exceedingly well. Characteristic fragments of various other 

 lamellibranchs, brachiopods, small gasteropods, echinoids. and 

 Crustacea are recognizable, but form only a small proportion of the 

 whole. A few of the fragments have become granular, but such as 

 were originally calcite have retained even the most minute of their 

 microstructures. Of other constituents a fibrous, yellowish-brown, 

 non-pleochroic mineral giving low-interference colours and straight 

 extinction, and occurring in shreds and plates, is the most abundant. 

 Some of it appears to show organic structure, and may, I think, be 

 chitin. Chips of clastic quartz occur sporadically, and one or two 

 prisms of fairly fresh orthoclase were observed. A few small and 

 irregular masses of isotropic or aggregate-polarizing glauconite were 

 also seen in the slice examined, and are probably much more abundant 

 in other parts of the rock. A groundmass is present, in small and 

 variable quantity. Much of it is calcite, and is in crystalline con- 

 tinuity with the adjoining shell-fragments, but a certain amount of 

 finely-granular material and irresolvable clay-paste occurs in the 

 interstices. Unfortunately, the slice does not happen to show any 

 of the phosphate-nodules. 



The fauna contained in the Hard Band is not markedly different 

 from that of the immediately-underlying clay, and in the following 



1 Slide No. 4308 in the Sedgwick-Museum Collection, Cambridge. 

 Q. J. G. S. No. 239. 2 b 



