EXPERIMENTS OF SIR JAMES HALL. 



147 



tion of calcareous rocks from animal remains, it is not necessary to 

 discuss at present : that a considerable portion of many limestone 

 rocks was so formed, cannot be denied. It was however objected 

 to this theory, that the well known action of fire on limestone rocks 

 would expel the fixed air, and render them soft and pulverulent. 

 To this objection it was replied, that as the action of central heat on 

 beds of marine shells took place under the ocean, the pressure of 

 the water would prevent the escape of the fixed air, and would prob- 

 ably render the calcareous earth more fusible. This answer was 

 regarded as a mere hypothesis for some time, but Sir James Hall 

 determined to try its validity by experiments. Having calculated 

 the resistance which a column of water fifteen hundred feet, or any 

 given depth, would present to the escape of fixed air, he enclosed a 

 quantity of powdered chalk in a gun barrel, and confined it in such 

 a manner as to present an equal degree of resistance. He subjected 

 the powdered chalk thus confined, to the action of a furnace ; after 

 some time it was drawn out and cooled, and was found convert- 

 ed into crystalline limestone or marble ; and in one instance, where 

 the chalk enclosed a shell, the shell had acquired a crystalline tex- 

 ture, without losing its form. Hence, in situations where chalk or 

 earthy limestone are found to have a crystalline texture, when in con- 

 tiguity with trap rocks, we may with a high degree of probability in- 

 fer, that the limestone had been fused by the trap. 



A recapitulation of the facts and experiments which prove the ig- 

 neous origin of trap rocks, would afford a mass of evidence which 

 might convince the most sceptical enquirer ; but such a recapitula- 

 tion is needless, as in many situations undoubted currents of lava 

 pass into trap rocks, and we have ocular demonstration of the fact. 



The reason why geologists were so long opposed to the igneous 

 origin of basaltic rocks, may be explained partly by the attachment 

 to received theories, and partly by the reluctance to admit a condi- 

 tion of our planet, so remote from present experience. It was thought 

 an ample claim on our credulity, when we were required to believe, 

 that all the habitable parts of the globe had been for ages submerged 

 in the ocean, without requiring the further belief, that countries now 

 remote from active volcanoes, had been repeatedly subject to the 

 agency of subterranean fire. Yet, both these positions must be 

 granted, if we will allow a legitimate induction from established facts. 



The advocates of the aqueous origin of basaltic rocks, while they 

 advanced theories, which made claims upon our faith, equally un- 

 supported by present experience, failed entirely, in their attempts to 

 explain, in a satisfactory manner, the causes of existing phenomena. 

 The theory of Werner was, for some time, zealously supported, and 

 particularly the least tenable part of it, — the formation of basaltic 

 rocks by a second rising of the ocean, which deposited them on the 

 summits of elevated mountains. It may be proper to give a brief 

 account of this part of the Wernerian system, before it sinks into 

 entire oblivion. 



