494 iV. Knight — Dolomites of Eautern Iowa. 



carefully heating to the boiling-point. The insoluble portion, whicli 

 is the silica, was filtered off, dried in an air-bath, and the weight 

 determined. 



(fc) A gram of the fine powder placed in a porcelain evaporating 

 dish of 100 c.c. capacity was treated with dilute hydrochloric acid 

 and covered with a watch-glass. It was warmed on the water- 

 bath until there was no further evolution of carbon dioxide. The 

 watch-glass was removed, and the dish was kept on the water-bath 

 until crystals began to appear. As the drying continued the 

 substance was constantly stirred with a glass rod, until a fine dry 

 powder resulted. The powder was next moistened with a few 

 drops of concentrated hydrochloric acid and 20 c.c. dilute hydrochloric 

 acid (equal parts concentrated hydrochloric acid and water), and 

 about the same quantity of water was added. The contents of 

 the dish were then filtered and the silica determined. 



The results for the two methods were as follows : — 



The treatment described under (6) would decompose a silicate, 

 while the method under (a) would not. As the two series of results 

 are faii'ly concordant, the conclusion is that the silica exists as a fine 

 sand disseminated through the rock, A private communication from 

 W. H. Norton, of the Cornell College department of geology, states 

 that he came to the same conclusion while studying the rock with 

 a petrological microscope. The method described under (a) is 

 simpler than (h), and the work can be done in a much shorter time. 

 It is therefore to be preferred in the analysis of rock of this kind. 



2. The condition of the iron. A gram of the substance was 

 introduced into a flask of 120 c.c. capacity, fitted with a bulb tube 

 and Bunsen valve to prevent oxidation of the iron. It was dissolved 

 in dilute hydrochloric acid. A few drops of the cooled solution 

 were then withdrawn with a capillary tube, placed on a watch-glass, 

 and tested with a crystal of potassium ferricyanide. No suggestion 

 of a blue colour resulted, showing the iron to be in the ferric 

 condition. This increases the value of a rock as a building material, 

 as ferrous carbonate is an unstable substance with a tendency to 

 change to the ferric condition. A complete analysis of the specimen 

 resulted as follows : — 



CaCog 53-62 per cent. 



MgCo3 * 44-96 



Si02 0-83 



Al,03 0-25 



Fe'sOs 0-34 



100-00 

 The specimen is nearly a true dolomite, which contains 54-35 

 per cent. CaCog and 45'65 per cent. MgCog. This method of 



