TRANSACTIONS OF SECTION C. 573 



the essential difficulties of the subject would, I fear, he a most difficult undertaking. 

 In the meantime all that we can do is to compare the structure of known artificial 

 products with that of natural rocks, and to draw the best conclusions we can from 

 the facts, as viewed in the light of our present knowledge of chemistry and physics. 

 My own impression is that there is still much to be learned respecting the exact 

 conditions under which some of our commonest rocks were formed. 



The following Reports and Papers were read : — 



Sixth Meijort on the GircuJation of the Underground Waters in the Per- 

 mian, New Bed Sandstone, and Jurassic Formations of England, and the 

 Quantity and Character of the Water supplied to towns and districts from 

 those formations. — See Reports, p. 87. 



2. Notes on the Submarine Geology of the English Channel off the Coast of 

 South Devon} By Arthur Roope Hunt, M.A., F.G.S. 



The author described and exhibited hand specimens of the large detached blocks 

 of serpentine, gabbro, conglomeratic grit, granite, hornblendic granite, and other 

 gi'anitic or gneissic rocks that are occasionally trawled by the Brixham fishermen 

 m the English Channel, oft' the southern headlands of Devonshire. From the fact 

 that the nearest known rocks on the north-west are the gneisses of the Eddj-stone, 

 and those on the north and north-east are the micaceous slates of the Start and 

 Bolt district, it would seem probable that the detached blocks indicate similar rocks 

 in sitit, and that they are not erratics. 



3, On the Action of Carhonic Acid on lAmestone. 

 By Professox' W. Botd Dawkins, M.A., F.B.S. 



Caves in limestone are to be looked upon as subterranean watercourses, which 

 are produced partly by the dissolving action of the carbonic acid in the rainwater, 

 and partly by the mechanical action of the streams flowing through the caves. The 

 insoluble carbonate of lime in the rock is changed into the soluble bi-carbonate, and 

 is carried away in solution. The additional atom of carbonic acid, however, is in a 

 condition of unstable chemical combination, and if it be removed, either by evapo- 

 ration or by the action of the free current of air, the insoluble carbonate of lime is 

 at once deposited. Hence it is that some caverns have their walls covered with a 

 drapery of stalactite, and the little straw-like pendants from the roof formed round 

 the edges of each drop gradually become developed into columns of various sizes. 

 The stalagmitic pedestals also rise from the floor where a line of drops falls from 

 the roof, and ultimately unite with the column let down from above. On the 

 surface, too, of the pools an ice-like sheet of stalagmite gradually shoots across 

 from the sides, and sometimes, where the water is still, covers the whole surface. 

 Admirable illustrations of all these processes are to be seen in the caves of Pem- 

 brokeshire, and especially in the Fairy Cave on Caldy Island. 



The rate of the accumulation of carbonate of lime, depending primarily upon the 

 access of water and the free access of air, both being variable, varies in diff'erent 

 places. Sometimes it is very swift, as for example in the Ingleborough Cave ', 

 where a series of observations by Professor Phillips, Mr. Farrar, and myself, extend- 

 ing over the years from 1845 to 1873, give the annual rate at '2946 inch. It is 

 obvious, therefore, that all speculation as to the antiquity of deposits, in cases 

 which are based on the view that the accumulation is A'ery slow, are without value. 



The narrow mountain-limestone ravines and passes are to be viewed, in the 

 main, as caverns formed in the manner above stated, which have lost their roofs 

 by the various sub-aereal agents which are ever at work attacking the surface 



'• See Transactions DevansMre Association for 1880. 



