560 J. G. Malcolmson, Esq., on the Fossils of 



smooth, without any impressions of the basalt, and is composed of a thick covering of cacho- 

 long, parallel stripes of which appear to indicate the slow cooling of the surface. The centre of 

 the mass is composed of quartz crystals, radiating to the centre, which is occupied by calca- 

 reous spar impressing or impressed by the quartz. The mode of its occurrence increases the 

 conviction in my mind, that the only correct theory of the formation of such minerals in trap 

 rocks, is the play of the molecular attractions existing between similar particles of matter. 

 That so eminent an inquirer as Dr. Turner should have asserted that all calcedonies, rock cry- 

 stals, &c., even when occurring in volcanic rocks, are the result of aqueous infiltration *, I can 

 only account for, by the difficulty of explaining how carbonic acid is retained at high tempera- 

 tures, in any other way than by supposing a great pressure to have existed at the time the rock 

 was in an ignited state ; but of the existence of which, proof is often entirely wanting where cry- 

 stals of carbonate of 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 ex- 

 posing 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 effect of aqueous 

 vapour in assisting the escape of carbonic acid from limestone, and concludes, that its agency is 

 trifling f ; 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 beautiful 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 import- 

 ant, in showing that the calcareous matter in a rock exposed to volcanic action may lose its water 

 before the carbonic acid, and be thus reduced to the state of the carbonate of lime in Mr. Fara- 

 day'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 tlie 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 ignited J. 



In stating these views, I venture only to express an opinion forced on me by the phenomena 

 under description, and which appear to explain some of them better than the theory of pressure, 

 which, however useful in removing the prejudices against the igneous origin of the trap rocks, 



* Lecture on the Chemistry of Geology, by Dr. E. Turner, Edin, New Phil. Journal, Oct., 1833. 



t Annales de Chimie et de Physique, Oct., 1836. 



I For a knowledge of the fact, that lime calcined with finely-divided silica, acts like the fixed 

 alkalis in rendering it gelatinous and soluble in weak acids, I am indebted to Capt. Smith of the 

 Madras Engineers, F.R.S. 



