596 W. A. TARE OOLITES IX SHALE AXD THEIR ORIGIN 



81), and that "colloidal silicic acid is believed to be deposited largely with 

 the clays and muds, as the same processes of flocculation which precipi- 

 tate the colloids also throw down the finely divided sediments" (page 102) . 

 The plasticity of shales is dtic to colloids and the principal colloid in them 

 is silica; so the view that the silica might assume the oolitic form at times 

 is qnite probable. That oolites are not more common in shales is due to 

 the lack of favorable conditions for the rapid precipitation of the colloids 

 and a lack of an abundance of colloidal silica. 



2. There are no residual grains of calcite or aragonite present in the 

 oolites. Even though aragonite is the form in which most calcium car- 

 bonate is precipitated, it is soon converted into calcite and would appear 

 as such in the shales. A few grains of calcite, very irregular in shape, 

 are found in the yellow and green shales. Some of these grains arc in the 

 oolites, but the majority are in the shale around them. Their form, dis- 

 tribution, and relationship to the surrounding material is such that to 

 assume that the grains in the oolites are residual is to assume that the 

 grains in the shale are also residual, and that the calcite has been replaced 

 by shale, an assumption no one would agree to. The calcite is original, 

 as are all the constituents of the shale. The writer ha&a large collection 

 of slides of calcareous and siliceous oolites from previously descril)ed and 

 new localities. Some of them show replacement by silica ; some are wholly 

 calcareous, and none of the siliceous oolites in shale show the character- 

 istics of those siliceous oolites which have replaced calcite. 



3. All silicified calcareous oolites show siliceous cementation. This is 

 never present in the oolitic shale. 



4. The imperviousness of clays and shales preclude any possibility of 

 the introduction of silica into the shales. All men recognize shales as 

 being impervious barriers to the passage of water in controlling artesian 

 water supplies, in ore deposits, in oil fields, and also to the passage of gas 

 in gas fields. The following extract from Leith and Mead testifies fur- 

 ther to the imperviousness of shales : 



"Argillaceous sediments, althougli very porous, are cliaracteristically imper- 

 vious and do not permit even moderately free circulation of solutions. Water 

 circulation depends on size and continuity of openings rather than on total 

 volume of openings or degree of porosity. Cementation by infiltration of 

 materials from extraneous sources is believed to be largely inhibited by the 

 imperviousness of clay to free w^ater circulation. Induration is probably ac- 

 complished mainly by compression and internal rearrangement of original con- 

 stituents" (ibid., p. 103). 



."). There is no adequate source of the silica with which to produce re- 

 placement. If it can not come from the outside, then only the minerals 



