162 Geological Society: — ■ 



of the Lake-distriot." By J. Clifton "Ward, Esq., E.G.S., of tho 

 Geological Survey of England and Wales*. 



Part I. On the Liquid-cavities in the Quartz-bearing llocls of the 

 Lake-district. 



Tho object of this paper was to examine into the evidence afforded 

 by the liquid-cavities of the granitoid rocks of the Lake-district, 

 in reference to the pressure under which these rocks may have 

 consolidated. In the first division of the subject the geological re- 

 lations of the three granitic centres of the district were considered, 

 and it was shown that these several granitic masses probably soli- 

 dified at depths varying from 14,000 to 30,000 feet. The most 

 probable maximum depth for the Skiddaw granite was stated as 

 30,000 feet, the maximum for the Eskdale granite 22,000 feet, 

 and for the Shap granite 14,000 feet. These maximum depths were 

 arrived at by estimating the greatest thickness of strata that was 

 ever, at one time, accumulated above the horizon of the top of the 

 Skiddaw slates. 



The mode of microscopic examination, together with a description 

 of the precautions taken in measuring the relative sizes of the 

 cavities and their contained vacuities, formed the second division of 

 the paper. It was stated that all the measurements used in the 

 calculations were made from cases in which the vacuity moved freely 

 in the liquid of the cavity ; and an approximately perfect case for 

 measurement was defined to be one in which the outline of the 

 liquid-cavity was sharply defined all round in one focus, and in 

 which the vacuity moved freely to every part of the cavity without 

 going out of focus. 



Then followed the general results of the examination. Restrict- 

 ing the measurements to such cases as those above mentioned, the 

 results were found to be generally consistent with one another, and 

 with those previously obtained by Mr. Sorby in his examination of 

 other granitic districts. From the fact that the calculated pressure 

 in feet of rock was in all cases greatly in excess of the pressure 

 which could have resulted from the thickness of overlying rocks, it 

 was inferred as probable that these granitic masses were not directly 

 connected with volcanic action, by which the pressure might have 

 been relieved, but that the surplus pressure was spent in the work 

 of elevation and contortion of the overlying rocks. 



Microscopic, combined with field evidence, was thought to indicate 

 that the Shap granite, though mainly formed at a depth similar to 

 that at which the Eskdale granite consolidated, was yet itself finally 

 consolidated at a much less depth, the mass having eaten its way up- 

 wards at a certain point, and, perhaps, representing an unsuccessful 

 effort towards the formation of a volcanic centre. 



The examination showed that the mean of the pressures under 



which the Lake-district granites probably consolidated was nearly 



the same as the mean which Mr. Sorby arrived at for those of 



Cornwall. In conclusion the author stated that he wished these 



* Parts I. and II, were read at the Meeting of June 23. 



