OOLITIC GRAINS; 9 
the microscopic examination of the residues left after treating the 
rock with acid. 
By an increase in the amount of excessively fine-grained 
argillaceous material, the compact limestones are connected with 
the clays ; and by an increase in the amount of coarser arenaceous 
material the shelly and oolitic limestones pass into sandstones. 
I will now consider the constituents of these rocks in greater 
detail dealing with them in the following order : I. Oolitic 
Grains; II. Organic Fragments; III. Non-calcareous Detrital 
Material; IV. Matrix. 
I. Oolitic Grains. 
Spherical or ellipsoidal grains (" rice-grains ") which vary in 
size from that of small shot to that of peas, and which consist for 
the most part of carbonate of lime, enter largely into the composi- 
tion of the rocks under consideration. These grains may be 
classified as follows : 
(1.) Rolled fragments of organic bodies. 
(2.) Amorphous pellets without any recognizable structure. 
(3.) Grains showing a rude concentric arrangement, e.g., 
the pisolites, in which Girvanella-structure is common.* 
(4.) Grains showing both concentric and radial structures with 
reference to one or more nuclei. 
(5.) Compound grains made up of oolitic grains, shell-frag- 
ments, &c. 
The investigation of the more minute structures of these grains 
under the microscope is attended with considerable difficulty, in 
consequence of the want of transparency in the grains. Extremely 
thin sections are required, and even then the structures are often 
very indistinct. 
The typical oolitic grain is one which shows both radial and 
concentric structure. In a very thin section such a grain will 
give an ill-defined black cross in polarized light. It is made up 
largely of minute particles or fibres of calcite, which have their 
principal axes roughly arranged in a radial manner with reference 
to the centre of the grain. The surfaces of some of these grains 
are seen to be rough in consequence of the projection of exces- 
sively minute crystals of dog-tooth spar. The indefiniteness of 
the black* cross proves that the radial arrangement of the axes 
of the crystalline particles is only approximately realized. 
That the typical oolitic grain, above referred to, is the result of 
growth by accretion appears certain. It is probable, however, as 
Dr, Sorby has pointed out,f that this growth is not a simple 
chemical process but a combination of chemical and mechanical 
processes, such as the crystallization of carbonate of lime and the 
mechanical picking up of foreign matter. There does not appear 
to be satisfactory proof that the growth of organisms of the 
* More particular accounts of" Girvanella," are given on pp. 15, 16. 
t Address to Geol. Soc. 1879 ; Quart. Journ. Geol. Soc., vol. xxxv. (Proc.) 
pp. 74, &c. See also Zirkel, Lehrbuch der Petrographie, Ed. 2, vol. i., 1893 pp. 
484-489 ; and Bleicher, Coniptes Rendus, vol. cxiv., 1892, p, 1138. 
