KANSAS ACADEMY OF SCIENCE. 25 



the architect. But instead of any such arrangement I found the mound to 

 be composed of horizontal strata of limestone, broken off sharply at the 

 edges, where sections of many feet in thickness are exposed. This limestone 

 surprised me by its extreme hardness, which was from four to five, and in 

 some places seemed to me to exceed the latter figure; indeed, so remarkable 

 was it in this respect, that I could hardly believe it to be a mere limestone 

 until I had applied a chemical test. Although it is a limestone, it is not an 

 ordinary one, as is evident from its physical appearance. It is exceedingly por- 

 ous and unhomogeneous, showing everywhere the results of chemical change 

 effected by some ingredient of the spring-water. That this change has con- 

 sisted largely in the formation of a hard, insoluble silicate, by metathetical 

 reaction between the sodium silicate of the water and the carbonate of the 

 limestone (or possibly of the water itself J, is evident from the physical ap- 

 pearance of the rock. The portions showing the greatest porosity are the 

 hardest, and in the larger cavities a hard layer of silicate can be distinctly 

 seen and easily identified as such. 



Although the silica of the water is in so small quantity as to be reported 

 as "traces," I think it perfectly competent to have effected this change, con- 

 sidering the long time it has had for its work. Moreover, it is the only 

 conceivable agent, under the circumstances. 



Being curious to learn what changes, if any, had taken place in the bases 

 of the original limestone, I selected a piece in which silicification had occur- 

 red but moderately, and submitted it to analysis, after having removed by 

 washing all the soluble salts deposited in its pores, with the following result: 



Lime, Ca 52.99 percent. 



Magnesia, MgO 1.32 " " , 



Ferrous oxide, Fe 1.07 " " 



Carbonic oxide, CO2 42.59 " " 



Silica, Si O2 2.03 " " 



100.00 



As dilute chlorhydric acid extracts a large part, if not all, of the ferrous 



salt, and as the hardness of the silicate observed in the rock is greater than 



that of ordinary hydrated magnesium silicate, I conclude that probably the 



magnesium and iron are mainly in the form of carbonates, and that the silica 



is combined chiefly with lime, thus : 



Calcium carbonate, Ca CO3 92.02 percent. 



Magnesium carbonate, Mg CO3 2.77 " " 



Ferrous carbonate, Fe CO3 1.72 " " 



Calcium silicate, (and silica?) < Q. Q ^^q « « 



100.00 



In this specimen the silicification had not been great. I presume speci- 

 mens could be found showing tv/o or three times as much silicate; and it 

 must be borne in mind that a silicate formed in the pores of a limestone by 

 infiltration and double decomposition, would have many times the hardening 

 and cementing power of the same amount of silicate mechanically mixed 



