62 L. M. Parsons — 



there is a steep gradient, whereas there is hardly any gradient in 

 the Masson and Upperwood material. 



Porosity has been determined in the following manner — a 

 moderately large specimen from each locality was thoroughly 

 dried and weighed. Then it was completely coated with a thin layer 

 of paraffin wax to prevent water entering the pores and again 

 weighed. The difference in the two weights gave the weight of the 

 wax. The specific gravity of the specimen was then found in 

 the usual way, the necessary corrections being made to allow for the 

 wax. Next the wax was removed by immersion in hot water. The 

 specimen Avas again dried, finely powdered, and the specific gravity 

 of the powder determined by means of a specific gravity bottle. 



W. CRATTON CREV WVN SCARP. JUCHOLE MASiON UPPER- £ 



TOR TOR Y/OOD 

 /o* /oo 



qo — — — — yo 



So ..^. .^—.j^" '^ ?c 



76. 1^ 70 



CA.i_/V/)x. 



60 ;;; 



Fig. 2. — Graphs illustrating variation in chemical composition, dolomitic gradients, and 

 porosity in the material between Gratton and Cromford. 



From the data thus obtained the porosity was calculated as a ^ 



percentage by using the formula : — ■ J 



Porosity = U=- 



where D = specific gravity of powder 



and d = specific gravity of original lump inclusive of pores. 



Experiments on porosity appear to be helpful in connexion with 

 the question of leaching by surface waters.^ The porous nature of 

 the dolomitic material compared with that of the transition 

 limestones indicates that extensive leaching has occurred in the 

 dolomitic parts only, and the inference is that the process of 

 dolomitization has so reconstituted the rock that the removal 

 of residual calcite has been thereby facilitated. 



^ The porosity is only partly explained by molecular contraction due to 

 dolomitization. 



