86 



ME. W. E. BALDWIN- WISEMAN ON 

 Table III. 



[Feb. 1907, 



No. of 

 specimen. 



Description of rock. 



Distance 



from dyke, 



in feet. 



Percentage 

 volume of 

 contamed 



water when 

 soaked. 



1. 

 2. 



3. 

 4. 

 5. 

 6. 



7. 

 8. 



Unaltered djke-rock, almost black 

 in colour 



27 

 30 



90 



6-88 



4-81 

 13-49 

 12-42 

 13-50 



308 

 2-29 



0-24 



Slightly-altered dyke-rock, between 

 bigh and low water, dark olive- 



Chalky, dull-white limestone, em- 

 bedded in dyke-rock 



Chalky, dull-white limestone, in 

 contact with dyke-rock 



Dense black, somewhat flaky, lenti- 

 cular limestone 



Dense black limestone, with little or 

 no tendency to break into layers 

 or prisms 



Do. do. do. 

 Dense black unaltered limestone, 

 with irregular conchoidal fracture 

 and no tendency to break into 





In another case, highly-fossiliferous Carboniferous Limestone from 

 the shore near Port St. Mary (Isle of Man), collected above high- 

 water mark, had a porosity equivalent to *0078 of the total volume: 

 while fragments broken from pinnacles of waterworn rock between 

 high and low water had a porosity of *0341, — an increase of nearly 

 350 per cent, in a distance of only 100 yards. 



It is at once evident that, when the possibility of the variation of 

 the porosity of a rock is of such a nature as any of the instances 

 previously quoted, one could not presume an uniform rate of flow 

 in such an area. In fact, one is not justified in assuming conditions 

 of uniform flow in any area, without substantiating it by rigorous 

 investigations in the field. Yet mathematical theories of flow are 

 formulated, which are based upon the fundamental assumption of a 

 homogeneous rock-texture: the results obtained being, as one would 

 expect, almost identical in every respect with that of the passage of 

 electricity through a conductor. Such a process possesses but little 

 accuracy or reliability, and not infrequently is at utter variance with 

 the data derived from field-studies. 



To arrive, therefore, at a reliable estimate of the rate of flow of 

 water through any rock, or its storage-capacity, something more than 

 these mere empirical assumptions is essential ; and investigations 

 in the field must be supplemented by the data derived from 

 laboratory-experiments, carried out with test-pieces of fair samples 

 of the rock of the locality. 



