1840.] 



of the Great Basaltic District of India, 



85 



lime occur. The phenomena exhibited by the vesicular trap, scoriae, and 

 porous chert associated with the Indian basaltic and fossiliferous rocks, 

 satisfied me, that at the time of their formation they were subject to no 

 pressure sufficient to retain the carbonic acid of the altered limestones, 

 and of the shells and calc spar inclosed in the geodes ; but no other ex- 

 planation presented itself, till I was informed by Mr. Faraday of his 

 beautiful experiment of exposing carbonate of lime in perfectly dry air 

 to the heat of the oxy-hydrogen blowpipe, without driving off its carbonic 

 acid. In the simple apparatus employed by him to show that the reten- 

 tion of the acid depended on the absence of moisture, no pressure of 

 any consequence could be exerted on the lime. Guy Lussac has lately 

 published some experiments on the eiEFect of aqueous vapour in assisting 

 the escape of carbonic acid from limestone, and concludes, that its agency 

 is trifling* ; but as he does not appear to have taken the precaution of 

 drying the atmospheric air by passing it through sulphuric acid, as was 

 done by Mr. Faraday, they cannot be considered as invalidating the beau- 

 tiful results obtained by the latter. His observation, however, that the 

 water contained in limestones is driven off before the carbonic acid and at 

 a much lower heat, is important, in showing that the calcareous matter 

 in a rock exposed to volcanic action may lose its water before the car- 

 bonic acid, and be thus reduced to the state of the carbonate of lime in 

 Mr. Faraday's platinum tube. These facts will assist in explaining the 

 anomalies observed in the fossils of the district above referred to, which 

 often retain their carbonic acid when portions of the rock in which they 

 occur have been fused by the inclosing basalt, while other portions are 

 vesicular from the escape of gaseous matters. The whole phenomena, 

 indeed, would admit of explanation by supposing, what must in fact have 

 occurred, the presence or absence of moisture during the various degrees 

 of heat to which the rocks were exposed in the progress of the eruption, 

 and in the course of cooling. A shell, in one part of a rock, may thus 

 retain its carbonic acid, while in another portion it may be reduced to 

 quick lime, subsequently carried off by the water, leaving only a cast ; 

 and a third may be replaced by silica, or the form of its convolutions taken 

 by fine quartz crystals, perhaps derived from silica rendered gelatinous 

 by the lime with which it was ignitedt. 



In stating these views, I venture only to express an opinion forced on 

 me by the phenomena under description, and which appear to explain 



* Annales de Chimie etde Physique, Oct., 1836. 



+ For a knowledge of the fact, that lime calcined with finel5''-divided silica, acts like 

 he fixed alkalis in rendering it gelatinous and solulble ia Weak acids, I am indebted to 

 Capt. Smith of the Madras Engineers, F, R, iS, 



