No. 2102. A PECULIAR OOLITE FROM BETnLEHEM— WHERRY. 155 



Although the rock has probably undergone some alteration since 

 its original deposition, it seemed worth while to make an analysis of 

 it, the results of which are given in column 1, above. This shows it 

 to be a high-magnesian limestone. It is not possible to separate the 

 Goids from the matrix in the fresh rock, but where slight weathering 

 has taken place the ground-mass has become soft and sandy, and 

 they stand out m relief and can readily be picked out. In the weath- 

 ering process they have no doubt been altered slightly, so that their 

 origmal composition is indeterminate, but an analysis, given in col- 

 umn 2, shows them to differ from the rock itself to a greater extent 

 than can be accounted for by weathering. 



Comparison of columns 2 and 3 in the table shows the ooids to be 

 higher in dolomite, quartz, kaolin, limonite, and carbon, and lower in 

 calcite and siderite than the matrix. 



That quartz and kaolin should be higher is to be expected, for 

 grains of these minerals acted as nuclei for the formation of the ooids 

 in the first place. That carbon should be higher is also normal, for 

 low forms of life no doubt took part in the deposition of the con- 

 centric coats of the carbonates. 



The greater amount of limonite is probably to be correlated with 

 that of carbon, for the former has been produced by the decomposi- 

 tion of pyrite, precipitated from circulating iron sulphate solutions 

 by the carbonaceous matter. The reason for the greater amount of 

 dolomite and smaller of siderite is discussed later. 



The significance of practically all of the above-described features 

 becomes evident when the probable mode of formation of the "half- 

 moon" oolite is considered. When the ooids were first formed they 

 no doubt consisted of aragonite, whereas the matrix was dolomite- 

 mud. Mixed with the aragonite, in varying amounts in the different 

 concentric layers, was the carbonaceous pigment. After the solidi- 

 fication of the sediment into rock and the development of joint cracks 

 (but before the uptilting of the beds) waters penetrated along these 

 cracks and along the bedding planes. Smce aragonite is more soluble 

 than the dolomite of the matrix, it dissolved away, leaving belimd 

 the carbon and the nuclei — sand grains and bits of kaolin — in some 

 cases stripped of aU concentrically deposited aragonite, in others still 

 retainmg a few layers. These settled to the bottom of the cavities 

 in heaps, the shapes of which varied with the sizes of the nuclei and 

 the stage in the solution process at which they fell into the masses 

 of carbon powder. 



At some later period water again traversed the rock, but this time 

 conditions were favorable to deposition instead of solution, and 

 secondary dolomite filled up aU openings in the rock, tension and 

 joint cracks as well as the holes left by the removal of the ooids. As 



