CONCKETIOXS IN SANDSTONES. 67 



Some of these acids also combine with silica to silico-azo-humic acids. 

 According to Mr. P. Thenard, acids of this series form spontaneonsly in the 

 soil from humic acid, the ammonia of rain water, the nitrogen of the air, 

 and the silica contained in the soil. 



Modus operandi. — It is clcar that a fragment of undeconiposed organic mat- 

 ter embedded in a porous sandstone ma)' decompose under the action of 

 percolating surface waters and that under favorable conditions it may yield 

 humic acid which will attack the magnetite, always present in greater or 

 smaller quantity. With the silica of the rock silico-azo-humic acid may 

 also be produced. Were large quantities of organic matter present to- 

 gether with much water the result might be a mere bleaching of the sand- 

 stone; but, if small quantities of the solutions of the humic compounds only 

 are formed, they will be drawn into the surrounding sandstone by capillai y 

 action and a more or less nearly spherical mass will be impregnated with 

 them. This mass may, perhaps, increase in size until the organic matter is 

 exhausted. 



The humic compounds are very unstable, and a globular mass, such 

 as is supposed above, would soon decompose into carbonic acid, water, etc. 

 There would then remain a spheroidal mass of carbonates and silicates of 

 the bases which had been dissolved at an earlier stage. The latter being 

 formed at low temperatures would not improbably be hydrous. Calcium 

 phosphate is soluble in solutions of carbonic acid, and one would therefore 

 expect to find the phosphorus of the organic substance also diffused through 

 the mass. The hypothesis of a decomposing organic nucleus thus appears 

 to account in a rational manner for all the observed facts. 



Summary of evidence. — Tlic fosslls occasloually uict witli lu saudstoue con- 

 cretions are so rare as only to suggest that these masses mav have been 

 indurated through the indirect action of organic matter. The presence of 

 phosphoric acid in notable quantities in the matrix of concretions which 

 contain no fossils greatly strengthens the hypothesis that organic matter 

 once existed in these masses, but has since disappeared. When it is found 

 that the chemical character of the matrix of these concretions is also such as 

 would result from the decomposition of organic matter by processes of which 

 the main features are well known, the weight of the concurrent evidence is 



