﻿178 DE. H. C. SOEBT OX THE APPLICATION OF i^^}' IQoSy 



Magnesian Limestone. 



There is so much uncertainty respecting many important parti- 

 culars connected with this rock, that I should say nothing about 

 it if it were not in the hope that it may help to clear up the 

 difficulties. One thing that seems fair]y certain is, that in South 

 Yorkshire, North Derbyshire, and Nottinghamshire, the rock was 

 not deposited as it now is. In many places the material has been 

 diifted along the bottom, but it is difficult or impossible to know 

 what would be the original angle of rest or the amount of the 

 interspaces. 



Many years ago I made a considerable number of chemical 

 analyses, and found that, as a general rule, the specimens con- 

 tained an excess of carbonate of lime above that which is found in 

 a true dolomite. I then concluded that in many, if not all, cases 

 this excess was due to infiltrated calcite. Sometimes there appeared 

 to be good evidence that the rock had been changed from a lime- 

 stone after deposition, but in others that dolomite-mud had been 

 deposited origiually. In the county of Durham the excess of carbo- 

 nate of lime in the Magnesian Limestone is great ; and it seems to 

 me that some of the exceptional concretionary structures seen there 

 may have been due, in part, to the original deposit having been to 

 some extent aragonite, afterwards segregated and crystallized as 

 calcite. 



As determined by the boiling-water method described farther on, 

 the empty spaces in the rock in South Yorkshire vary from 9 to 29 

 percent., and in one case were increased from 11 to 21 per cent, 

 by the action of dilute acid, perhaps due partly to the removal 

 of infiltrated calcite. On the whole, these values do not differ 

 materially from those found in rocks of Oolitic age, and, like them, 

 indicate no great pressure. 



I have fairly-complete particulars of two cases near Conis- 

 borough. In one at Crookhill the angle of a drift-bed is now 18^. 

 If originally the material were of moderate grain, the angle of rest 

 would have been about 34:° : this would indicate a contraction 

 from 100 to 48. The cavities might have been originally about 

 48 per cent, and are now about 8 per cent., so that, as indicated 

 by them, the contraction has been to about 56-d of the original. 

 This indicates a removal of 8 or 9 per cent, of solid matter. In 

 a good case at Cadeby the angle is 20^, but the cavities amount 

 to 27 per cent. : this indicates a contraction of 54 per cent., and 

 the removal of 17 per cent, of solid material. Taking the mean of 

 the four determinations, it should appear that the rock is now 

 about 52-5 per cent, of its original thickness and that 13 per cent, 

 has been removed ; these, hcrwever. must be looked upon as onlv 

 rough approximations, because the original character of the rock 

 is unknown. 



