CONCRETIONS IN SANDSTONES, 



05 



Loss at 100°, H'O 



Carbonic anliydi iilc, CO' . . 



Silica, SiO' 



Phosphoric acid, P-O" 



Alumina, AI'O^ 



Ferric oxide, Fc'O^ 



Slanj'anous oxide, MnO ... 



Lime, CaO 



Magnesia, ilgO 



Soda, Na'O 



Potassa, K'O 



Silica extracted by Na'CO^ 

 Kcaidue 



First 

 sample. 



0.855 

 8.052 

 0.3C2 

 0.034 

 0.32G 

 0. 007 

 0.195 

 11. 152 

 0.315 

 0.031 

 0.068 



Total perc 



Second 

 sample. 



0.1T7 

 0.035 

 0.385 

 0.783 

 0.162 

 0.139 

 0. 102 

 0. 122 

 0.119 

 2.865 

 72. 110 

 99. 956 



When the atomic ratios of this analysis are calculated it appears that 

 the protoxide bases found in the first sample are slightly in e.xcess of the 

 amount required to saturate the carbonic acid. All excess of these bases, 

 as well as the alumina and the iron (which is wholly in the ferric state), 

 must be combined with phosphoric and silicic acids. Assuming tliat the 

 phosphorus is as usual combined as triphosphate of calcium, the remainder 

 is a silicate or a mixture of silicates. The atomic ratio of the sum of these 

 silicates is: 



Si : fi" : R" 0.22688 : 0.09385 : 0.22695 



or Si : ft" : R" 5:2: 5. 



If the water is taken into account, as it apparently .should be, the ratio 

 becomes 2:5:2:5, indicating some form of a hydrous subsilicate. 



The cementing material of the concretion then is an intimate mixture 

 of calcium carbonate with a hydrous subsilicate and a small but not incon- 

 siderable amount of phosphate. Tlie subsilicate may probably be regarded 

 as an iron compound in which a portion of the iron is replaced b}' alumina 

 and protoxide bases. How such a cement can be formed within the body 

 of a sandstone is evidently a question of great interest; and in its answer 

 lies the secret of the class of concretions of which the specimen investi- 

 gated is an example. 



Action of a nucleus. — Siuce it Is iiiaiiifestlv impossible that these concretions 

 should be Imilt up during the deposition of the sand, they must lie due to 

 JVION xui— — 5 



