404 Miss E. Hodgson — On the Furness Limestone. 



It is manifestly impossible to claim any distinction for the Birk- 

 rigg rocks, however curious : they have their true imitations on the 

 high grounds of Grange, Silverdale, Wharton, Whitbarrow, and east- 

 ward on the Scars, near Kendal, Indeed, it is needless to multiply 

 examples, for is not this the natural appearance of the surface of the 

 Mountain Limestone wherever it is the upper rock of the country 

 and undisturbed? 



The limestone terraces of the Burren hills in Ireland do not appear 

 to have suggested the sea as the direct agent to the author of the 

 " Student's Manual," — the exposed beds are there " cut into blocks 

 by deep fissures, the uppermost blocks are loose and tottering,— and 

 are worn into rough knobs and holes hy the mechanical and chemical 

 action of the weather."^ 



The late Dr. Woodward, in a letter dated August, 1863, says, "It 

 has always appeared to me that the drip and splash of rain and rain- 

 water was the chief agent in honey-combing limestone hy virtue of its 

 dissolved carbonic acid gas." 



In "Principles of Geology,"^ due weight is given to the power of 

 carbonic acid in dissolving limestone rocks. 



Mr. Cameron, F.C.S. (to whom I sent specimens of rocks and clays 

 from Tarn Close and Birkrigg), points to the same chemical action 

 for their elucidation. He reminds me that carbonic acid gas is one 

 of those agents in nature that works slowly, but gradually and surely ; 

 that it decomposes the hardest rocks, carrying away part of their 

 constituents ; that it combines with potash, soda, lime, magnesia, 

 etc., water, the necessary medium by which it operates, absorbs its 

 own bulk, or rather more of it, from the atmosphere, and carries it 

 into fissures and crevices of the rocks, where it acts as a solvent upon 

 these substances, forming carbonates and bi-carbonates. 



With the Birkrigg, etc., rocks, no doubt rain-water, charged with 

 carbonic acid, has had a great deal to do. 



Mr. Cameron attaches considerable importance to the decaying 

 vegetable matter of the soil. Liebig states that humus (decayed 

 woody fibre) is a continual source of carbonic acid, — an atmosphere 

 of carbonic acid surrounds every particle of decaying humus ; ^ and, 

 again, "It is evident that plants, by producing carbonic acid during 

 their decay, and by means of the acids which exude from their 



1 "Manual of Geology," by J. Beete Jukes, p. 513. — Since writing the above, it 

 occurred to me to turn to the memoir by Mr. Jukes on the River Valleys of the 

 South of Ireland ; and I fear the fact will scarcely be believed, that the description 

 given there of the limestone of the Burren hiUs, so true to nature as it evidently is, 

 had hitherto entirely escaped my attention. 



2 Prin. of Geol. vol. i. pp. 331, 333, Sir C. Lyell. 



3 Liebig's Chemistry of Agriculture and Physiology, p. 48. There is palpable evi- 

 dence of the truth of this in the small angular stones (one cannot call them pebbles 

 or shingle) which may be found entangled amongst the grass-roots where the sod 

 is thin. It will be seen that there is a remarkable tendency amongst these to termi- 

 nate in points, more or less sharp ; sometimes in one, awl-like ; or in two or more, 

 often closely resembling fossil teeth of fishes. Below, where these occur, are quanti- 

 ties of miniature " rockery stones" of every form and shape. There could not, 

 certainly, be a greater proof, I think, of the absence of wave-action than in these 

 extraordinary examples of limestone d6bris. 



