262 American Association. 



admitted to explain tlie cliauges which they have been found to 

 have undergone, from the fact that such a temperature was incom- 

 patible with the existence of alkaline silicates and of graphite in the 

 limestone. The influence of hot water alone is equally inadmissible, 

 for the silica being dissolved by water before it could act upon the 

 bases, we should find the quartzites rendered vitreous and crys- 

 talline. 



He regards the changes as having been produced by the action 

 of small amounts of carbonate of soda in aqueous solution form- 

 ingwith the quartz, silicate of soda, which is afterwards decom-- 

 posed by the earthy carbonates ; yielding silicates of these bases, 

 and reproducing the carbonate of soda. A portion of the alkali 

 is, however, always fixed and rendered insoluble in the process, so 

 that with a limited portion of soda, the action is at last exhausted. 

 These reactions, resulting in the production of silicates of lime, 

 magnesia, &c., take place even at 212 "^ F, and the intervention of 

 alumina gives rise to garnet, chlorite and epidote. The absence 

 of iron from some felspathic and quartzose sediments, and its accu- 

 mulation as beds of iron ore, he regards as effected by the agency 

 of organic matters, which reduce the iron to protoxyd and render it 

 soluble in w^ater, which afterwards deposits it as oxyd or carbo- 

 nate. The same process produced the fire-clays and ironstones of 

 the coal period, and is now operating in bogs and marshes. In 

 this way we have beds of argillaceous and felspathic materials freed 

 from iron. 



CRYSTALLINE ROCKS Oi' THE NORTH HIGHLANDS OF SCOTLAND. 



We give in full a paper on this very interesting subject, com- 

 municated by Sir E. Murchison and Mr. Salter. It brings within 

 the limits of the recognised subdivisions of the Silurian System in. 

 Britain and America, a group of metamorphosed rocks, hitherto 

 of uncertain age. 



London, July 27, ISoT. 



My Dear Sir William, — Being unable, to my great regret, to 

 attend the Montreal Meeting of the American Association for the 

 Advancement of Science, where my distinguished friend Professor 

 Ramsay, will represent British Geologists and our Survey, I beg- 

 to communicate to you, and any geological contemporaries who 

 may be present, the final determination of a question which has 

 been much agitated in this country, and which has just been set- 

 tled by a comparison with North American typical fossils of 

 Lower Silurian age. This question is: what is the true place in 



