part 3] CHELLASTON GYPSUM BRECCIA. 20& 



closely similar deposits of the North -East of England. Those of' 

 the Trias, on the other hand, would seem to be due to the desicca- 

 tion at a lower temperature of inland lakes, which very probably 

 owed the materials they held in solution to wind transport across, 

 the desert, in the manner demonstrated by Sir Thomas Holland in. 

 the case of the Sambhar Lake. 



Mr. J. F. N. Gree:s t asked whether there was any evidence that 

 the ' pillars ' had formed as actual pillars on the lake-floor, like 

 those found now in certain salt-lakes— as, for example, those of 

 Mono Lake (California), where the structure was developed in con- 

 centric layers, sediment afterwards filling up the gaps. Or was. 

 chemical and sedimentary deposition simultaneous on a level floor ? 



Prof. W. J. Sollas thought that this excellent piece of petro- 

 logical work might prove to have an important bearing on other- 

 branches of geological study. The alternations of rock-salt and 

 anhydrite at Stassfurth had already suggested seasonal changes 

 from' summer to winter, and these could be counted, so that it was- 

 possible to determine the rate of deposition. In the alternations of 

 anhydrite and gypsum an even more definite record of changes in< 

 temperature and other conditions might, perhaps, be discovered. 



Sir Hexrt Ho worth and Mr. W. Whitaker also spoke. 



The Author, in reply to Dr. Evans, pointed out that some of" 

 the seams that provided the strongest evidence of original deposition, 

 occurred in the Permian rocks of the Yale of Eden. He was aware - 

 that conversion of anhydrite into g}^psum had occurred in Germany, 

 but evidence was not always forthcoming that the whole of each 

 bed of gypsum was formed in this way. 



The carvings made from Derbyshire and Staffordshire alabaster* 

 were, as Sir Henry Ho worth stated, justly famous, and of exceeding 

 beauty. Dr. Plot records that in the first half of the 17th Century,, 

 a pure white alabaster, watered with a bluish colour, was highly 

 prized by sculptors. This rock was similar to the blue-veined 

 alabaster quarried by Mr. Forman. The best white rock used in> 

 the past probably came from isolated pillars. 



An interesting point was raised by Prof. Sollas. The Author 

 had indicated recently how the 'skerry' bands or belts of the 

 Keuper, or the laminated silts and shales of the Ordovician and 

 Silurian, might one day furnish time-scales by which the duration 

 of these respective periods might be measured. 



Replying to Mr. Green, he did not think that the pillars 

 stood out above the surrounding sediments during formation ; their 

 upper surfaces, however, might have been slightly convex. There 

 was little difference between the gypsum at the surface and that 

 under cover. 



In conclusion, his thanks were due to Dr. H. H. Thomas and 

 Dr. F. L. Kitchin for their kind help during the preparation of' 

 the paper. 



