154 PROCEEDINGS OF THE NATIONAL AlUl^EUM. vol.49. 



In one layer about 20 cm. in thickness, which has been removed 

 from much of the quarry face, but is still (at time of writing) exposed 

 in place at the extreme north end, the majority of the ooids show, 

 when broken across, a remarkable "half -moon" aspect, being divided 

 parallel to the bedding into alight and a dark portion, the latter being 

 the lower. In most of them the light portion is the larger, but in 

 rare instances the dark may occupy nearly the entire grain. 



When exammed imder the microscope the dividing line between 

 the two parts is found to be, as a rule, convex toward the white one 

 but in a few mstances it is straight across, irregidarly curved, or even 

 slightly concave. The two parts are alike m crystallinity, both bemg 

 much coarser than the ground-mass of the rock; the black pigment 

 may be seen to spread in fdms between the individual crystals as 

 well as to be enclosed by them. The nuclei occasionally remaining 

 are no longer in the centers of the grams, but always well toward the 

 bottom of the dark portions. The tiny specks of limonite are prac- 

 tically absent from the white portions, but are often present all the 

 way around the dark ones. 



Tlu'oughout the bed hi which these "half -moon" ooids occur there 

 are lens-shaped areas in which the grahis are of the ordinary type. 

 These lie in the centers of blocks formed by the intereection of bed- 

 ding and joint planes. The ooids in them are also mostly recrystal- 

 lized, but as a rule are not as coarse-grained as the divided ones. 



Table of analyses. 



1. 2. 3. 



CaO 33.10 30.40 



MgO 17.84 19.71 



FeO 0.25 0.11 



CO2 45.70 45.57 



AI2O3 0.23 0.32 



FejOs 1.33 1.90 



SiOo 0.52 0.G8 



H26 0.32 0.44 



C 0.79 1.21 



100. 08 100. 34 

 Calculated mineral compositions. 



Dolomite 81. 05 



Calcite 14. 78 



Siderite 0.40 



Kaolin 0. 58 



Quartz 0. 25 



Limonite 1-55 



Carbon 0. 79 



100. 00 100. 00 100. 00 



1. Composition of the oolitic limestone. 



2. Composition of the ooids which have weathered out. 



3. Calculated composition of the matrix, assuming that the ooids make up about 



half the rock. 



