Miss E. Hodgson — On the Furness Limestone. 405 



roots in the living state, contribute powerfully to destroy the co- 

 herence of rocks. Next to the action of air, water, and change of 

 temperature, plants themselves are the most powerful agents in 

 effecting the disintegration of rocks." ^ 



Mr. Cameron thinks that the fine clays found in contact and also 

 in crevices and pockets may possibly in some cases represent the 

 argillaceous or insoluble portion of the dissolved rock itself. ^ 



The Furness limestones then seem to offer illustrations of three 

 somewhat different dissolving processes. Firstly, that of atmospheric 

 disintegration aided by frosts and direct rain-fall, and also by minute 

 encrusting lichens. (These influences are well shown on many 

 exposed rock-sui-faces of this district.) Secondly, that of rain and 

 decayed vegetation, as illustrated by rocks with a thin covering of 

 soil and drift. Thirdly, that of rain-water acting at greater depths 

 upon limestone rocks deeply covered with drift ; the more easily solu- 

 ble portions of the limestone being dissolved in every case. Assum- 

 ing the correctness of this explanation, it seems needless to call in so 

 remote an agent as the sea, even if it were capable of producing such 

 an effect. The striated and polished surfaces here and there met with, 

 which appear to be a few remnants of the Glacial Period, may have 

 been preserved from honey-combing and disintegration because their 

 polished and compact surfaces were better able to resist the chemical 

 forces attacking them, than the naturally pitted and uneven surface 

 of the rock elsewhere. 



It was in reference to the assumed work of MoUusks in rocks that 

 Dr. Woodward called my attention to the more likely agent of rain- 

 water ; and it has been my regret that I did not earlier perceive the 

 force and value of his suggestions. " In Gloucestershire," he writes, 

 " there is a bed of the great Oolite, called ' Dagham Down Stone,' 

 because it forms the substratum of a large extent of (formerly) down- 

 land. This must have originally enveloped a continuous bed of 

 sponges, or something of the kind, for now the rain eats into it 

 irregularly, leaving holes such as one could make with the fingers in 

 dough." 



1 have reason to believe that many of the cavities in the Birkrigg 

 stone may be traced to the former presence of fossils. The matrix 

 of the bivalve shell is recognized so long as its striee are preserved ; 

 but when these are effaced, the origin of the hollow becomes doubt- 

 ful. So it may be with the tubular cavities. 



A-t the same time it must be admitted that surface inequalities and 

 honey-combing, if not, indeed, every other form of disintegration, are 

 mainly due to the texture of the rock itself, inducing unequal 

 weathering, independently of any fossils. 



With regard to those minute punctures, sometimes accompanying 

 a lichen encrustation, and sometimes observed apart, it is probable 

 that the latter, as well as the former, owe their presence to the 

 saucers, or fruit receptacles of these plants. We have it on the autho- 



' Liebig's Chemistry of Agriculture and Physiology, p. 139. 



2 See Mr. George Maw's papers in the Geological Magazine, 1865, vol. ii. p. 

 200; 1866, vol. iii. p. 253; and 1867, vol. iv. pp. 241 and -299.— Edit. 



