34 DE. TH05IAS STEEEY HUNT ON THE 



nite itself may also be artificially effected.'" Palagouite, is not, apparently, a distinct mineral 

 species, but a colloidal liydrated mixture, interesting as marking a stage in the transforma- 

 tion of the vitreous form of certain basic silicated compounds. The crystalline forms of 

 these by their decomposition may, however, yield zeolites without passing through this 

 intermediate stage. 



§ Q^. That in these curious but neglected observations of Bunsen's, we have repro- 

 duced in miniature not only the process which takes place on the large scale in masses of 

 basic exoplutouic rock, but the process which must have gone on in the early ages, when 

 the universal basic rock, which we have supposed to form the surface of the cooling globe, 

 was heated from below, and penetrated by atmospheric waters — was a deduction which, 

 although it seemed legitimate, was too vast and too far-reaching to be lightly accepted. It 

 was therefore not until after many years of careful consideration, and the examination and 

 rejection of all other conceivable hypotheses, that the conviction was acquired that in 

 these reactions, which give rise to zeolitic minerals, we have the true solution of the pro- 

 blem of the genesis of crystalline rocks. This was formally enunciated in 1884, when, 

 after considering the condition of a cooling earth, in accordance with the hypothesis 

 defined in § 50, it was said : " The globe, consolidating at the centre, left a superficial 

 layer of matter, which has yielded all the elements of the earth's crust. This last-cooled 

 layer, mechanically disintegrated, saturated with water, and heated by the central mass, 

 furnished in aqvieous solïition the silicates which were the origin of the ancient gneisses 

 and similar rocks." ^ 



^The following is the composition assigned by Bunsen to the typical trachytic and basaltic magmas, and to 

 palagonite, as deduced from his studios of these rocks in Iceland ; A, being the normal trachytic type, the mean of 

 seven analyses of trachyte and obsidian ; B, the normal basaltic tyj», from six analyses of basalt and lava ; and 

 C, the average of several palagonites of that region, deducting the water : — 



A. B. C. 



Silica 76.67 48.47 49.15 



Alumina 11.15 14.78 -> 



Ferrousoxyd 3.07 15.38 |30.82 



Lime 1.45 11.87 9.73 



Magnesia 0.28 6.89 7.97 



Potash 3.20 0.65 0.99 



Soda 4.18 1.96 1.34 



100.00 100 00 100.00 



The ferrous oxyd in the six examples from which B was deduced varied from 11.09 to 19.43 ; while for the 

 palagonite, the iron (which is not separated from the alumina in the above average, and is present as ferric oxyd,) 

 ranged from 11.85 to 21.30. The water therein varied from 16.0 to 24.0 per cent. The oxygen ratio for pala- 

 gonite, taking the maximum of alumina, 18.97, and the ferric oxyd, 11.85, together, would be about 1:2:4- and 

 excluding the latter from the calculation, very nearly 1 : If : 4. Palagonite, according to Bunsen, is thus a 

 hydrated basalt which has exchanged a portion of its lime for magnesia, with peroxydation of the contained 

 iron. It " is the amorphous portion of basalt that gelatinizes with acids, which is the part forming zeolites " 

 (corresponding to the vitreous matter of tlie tachylite-basalts), and the liydration of this yields palagonite. 

 Bunsen, by fusing a basalt with potassic hydrate, and treating the mass with water, got a material which 

 differed from the basalt only in having lost a little silica and acquired 30.0 of water, and which had all the charac- 

 ters of palagonite. (Bunsen, Recherches sur la formation des roches volcaniques en Islande. Ann. de Chim. et 

 de Phys. (1853) (3) xxxviii., 215-289. 



^ From a report of a lecture by the author before the Lowell Institute, Boston, Mass., Feb, 29, 1884, in the 

 Boston Daily Advertiser of March 1. 



