GEOLOGY. 281 



among the nondecomposed forms are, more or less, acted upon when they 

 are reduced to a fine powder and exposed to a long digestion with concen- 

 trated acids. But in the decomposition of silicates which acids attack with 

 difficulty, silica is always precipitated in a pulverulent and not in a gelati- 

 nous form. From the different states in which silica is separated by the 

 action of acids, Bischoff considered that it exists in two isomeric conditions. 

 To test this theory, M. Rose prepared pulverulent silica from stilbite, with 

 the aid of concentrated hydrochloric acid, and found its density when dry 

 at 150 3 was 2.145; after a little water left at this temperature was expelled 

 by slight calcination, the density was 2.1897, and after half an hour's expo- 

 sure to a red heat, it was 2.200. Gelatinous silica was then obtained with 

 hydrochloric acid from apophyllite, and possessed a specific gravity of 2.218, 

 and, after calcination for half au hour near a reddi.sh white heat, of 2.22. 

 It therefore appears that silica possesses the same density and the same 

 properties in whichever condition it is separated from its combinations by 

 acids. The envelops of infusoria are formed of amorphous silica, do not 

 polarize light, and have, according to Schaffgotsch, a specific gravity of 2.2. 

 Thus, while the silica resulting from inorganic changes effected in nature by 

 the humid way is of a specific gravity of 2.G, the phenomena of organic 

 life transform this into silica with a densit3" of 2.2. 



The original paper further presents a mass of valuable detail which can- 

 not here be reproduced, after which the writer applies his researches to the 

 theory of the production of granite, and lays stress upon observations 

 tending to show that the quartz was crystallized after the felspar and mica, 

 whereas it would have been the first to crystallize if all had been fused 

 together, because it is the least fusible, and would solidify soonest. By 

 melting granite, M. Rose obtained a black glass mingled with portions of 

 silica. Various other reasons are given for reviving the Neptunian theory; 

 and the writer concludes that it is possible that the constituent elements 

 of granite mav have been formed bv the action of water on a mass origin- 



*j . 



ally in fusion, and considers that the high temperature and circumstances 

 under which the action took place would account for the absence of organic 

 remains. It is obvious that this theory affects not only granite, but porphy- 

 ries and other supposed igneous rocks. M. Rose, however, admits that if 

 any one should succeed in forming crystalline silica by fusion which should 

 possess a density of 2.0, the principal objections against the igneous 

 origin of granite would disappear. 



s 



ON THE FORMATION OF MINERALS IN THE HUMID WAY. 



In a communication on the above subject recently made to the Boston 

 Society of Natural History by Dr. C. T. Jackson, he remarked of chemical 

 springs, that they are generally found along the line of disruption of strata 

 of rocks, and near the junction of eruptive rocks with those of aqueous 

 deposition. In the Vosgcs, it is at the line of contact of granite and the new 

 red sandstone that the hot springs of Plombie'res are found. The waters of 

 these springs have a temperature of seventy-three degrees Centigrade, or 

 one hundred and sixty-three degrees Fahrenheit. These waters contain 

 0.03 grammes of silicate of potash per litre. Ancient Roman baths were 

 found at these springs, and the river had been turned out of its natural 

 channel into an artificial one, in order to accommodate the construction of 

 the baths. In these ancient works were found bronze stopcocks, in which 



24* 



