244 ME. W. HILL ON THE MINUTE STRUCTURE 



whether any particular sample was part of an outer reef, formed 

 under the lash of the breakers, or whether the materials were de- 

 posited in the quieter waters of the inner edge, in lagoons, or on 

 the beach, but the limestones examined present marked differences 

 of structure which are probably due to variation of the conditions 

 under which they accumulated. 



Examined in thin sections under the microscope they present 

 four varieties of structure : — 



1. Rock consisting of fragments, generally recognizable as 



coral, with many portions of a nullipore, set in a matrix 

 which is now finely granular calcite. No foraminifera. 

 (PI. IX. fig. 1.) 



2. Eock formed of moderately small particles, more or less 



closely packed, of a previously consolidated material, 

 with many portions of a nullipore and a few foraminifera, 

 the whole embedded in a matrix which is now granular 

 calcite. (PI. IX. fig. 2.) 



3. Hock consisting of the fragments of a considerable variety 



of calcareous organisms. The matrix is now either 

 granular or crystalline calcite. (PI. IX. fig. 3.) 



4. Eock consisting of definite separated grains cemented by 



pure crystalline calcite. 



(a) Largely formed of rounded fragments, exhibiting either 

 definite organic structure, or that of a previously con- 

 solidated calcareous mud. (PI. IX. fig. 4.) 



(h) In which the tests of Amphistegina are more numerous 

 than the rounded grains. (PL IX. fig. 5.) 



In the first variety of the coral-limestone the fragments which 

 are angular, and show for the most part a true coral-structure, are 

 set in a matrix of calcareous mud, and form as it were a fine breccia. 

 They are often perforated by minute tubules similar to those made 

 by boring algae in recent corals. 



The original character of the mud is entirely obscured by the 

 minutely granular crystalline structure which has been superinduced 

 in this matrix by the infiltration of calcite ; but I have found that 

 fine calcareous muds, which have become crystalline, always present 

 a similar translucent and finely granular structure when seen 

 in thin sections under the microscope, and it appears to be the 

 usual condition of such mud when infiltrated by crystalline carbonate 

 of lime. Portions of a nullipore (a Litliotliamnion), usually showing 

 an unbroken and continuous bounding edge of growth, form very 

 prominent objects, and occupy a considerable area in the sections. 



There are no foraminifera, and the structure of no other cal- 

 careous organism than coral and LitliotliamnioR appears in the 

 specimens which I have examined. 



Of this class there are two specimens, one from the surface 300 

 feet above sea-level at Bannatynes, and another from a quarry 700 

 feet above sea-level at Groves. 



This last specimen contains branches of coral varying from half 



