:24G MR. W. niLL ON THE MINUTE STRUCTURE 



with calcite, in some cases pure and crystalline, in others granular ; 

 but nearly always the infilling material is in coarser crystals than 

 the surrounding matrix, and the outline of the original fragment is 

 to a certain extent preserved. 



Specimens showing the structure of the second division come 

 from three localities. There are several from Castle Grant, where 

 the surface is over 1000 feet above sea-level, one from a shaft at 

 Lightfoots and another from a shaft at Bath ; the mouth of the 

 latter is 400 feet above sea-level, and the specimen was obtained 

 about 90 feet down. There is no important difference in any of 

 these specimens. 



The striking feature in the structure of the third description of 

 coral-limestone is the variety of the organic fragments of which it 

 consists. There can be recognized pieces of coral, nullipore and 

 shell, echinoid spines, plates, and ossicles, many species of forami- 

 nifera, amongst which AmjyJiistegina is common, and lastly portions 

 of polyzoa ; the whole rock is in fact a collection of the fragments 

 of the many organisms which exist on the coral-reefs. 



The size of the fragments differs as usual in the different speci- 

 mens of rock of this character ; in some of them a part may be 

 too small for identification ; in others, where the fragments are larger, 

 the structural details of each fragment are often well shown. 



The matrix varies ; in two specimens it appears to be fine cal- 

 careous mud, but in three others the material investing the frag- 

 ments is pure crystalline calcite. 



As before, the rock is full of cavities ; their outline often leaves 

 little doubt that they have resulted from the solution of one or other 

 of the enclosed fragments. Whether it is some particular class of 

 fragments which first disappear, or whether the solution of frag- 

 ments is accelerated or retarded by physical causes which determine 

 the course of percolating water, I cannot say. 



There are five specimens which show the structure of this 

 division. Two are from Plumtree Gully from a shaft, at about 

 45 feet below the surface ; a third is from a boring at Lightfoots ; a 

 fourth is from another shaft at Plumtree Gully, 30 feet from the 

 surface ; and the fifth from No. 3 Shaft, Eock Dundo, the mouth of 

 which is 303 feet above sea-level, and the specimen was obtained 

 44 feet from the surface. The first three closely resemble each 

 other, and they are the softest and least crystalline of the whole 

 series. The organic fragments are fewer and smaller than in the 

 two succeeding specimens, and the matrix of fine calcareous mud 

 is less altered than usual, the infiltration of crystalline calcite 

 being hardly complete, but in this respect the specimens show some 

 variation. In the next two the fragments are coarser, the cementing 

 material is chiefly crystalline calcite, but there are inclusions of cal- 

 careous mud ; the whole is, however, thoroughly infiltrated with 

 calcite, and is hard and crystalline. 



Of the foraminifera seen in these rocks Amphistegina Lessonii is 

 a prominent form. 



In the fourth division the rock is practically a coral-sand 



