Correspondence — H. C. Sarr/enf. 139 



4 feet per mile from Blackstone Edge towards the Irish Sea ; there- 

 fore, when at its maximum the ice-sheet was probably over 2,000 feet 

 above present sea-level in the middle of the Irish Sea in this latitude. 



Extensive systems of glacier-lakes and drainage-channels Avere 

 produced on the retreat of the ice, and for some time the drainage 

 on the west of the Peiiniiies in the Ribble and Irwell basins escaped 

 eastwards into the Yorkshire Calder. 



It is probable that the north-western ice arrived in this area later, 

 and disappeared earlier, than the Ribblesdale ice. 



Some local fluctuations in the ice-sheet occurred, but there is no 

 evidence for more than one Glacial period. 



ooi^i^ESiE'oisrnDEiisrcE. 



EOCK-SOIL AND PLANT DISTRIBUTION. 



Sir, — Mr. A. R. Horwood, in liis article under the above heading 

 in the January number of the Gkological Magazine, makes certain 

 statements in regard to the distribution of plants, with special 

 reference to Derbyshire, which must be accepted with caution. 



To deal only with the fourteen plants which he names as being "so 

 strong [in] their attachment to lime soils and their absence so well 

 marked upon others", that limestone could be mapped, even if only one 

 or two of them occurred where no exposures were available, I find 

 in Linton's Flora of Derbyshire ( 1903) that one only ( Galium asperum) 

 is peculiar to limestone ; seven are cliaracteristic of that formation, 

 but not peculiar to it; and the remaining six appear to occur 

 indiscriminately on all the formations in the county. 



1 happen to live in Derbyshire, in the immediate neighbourhood 

 of Carboniferous Limestone, Limestone Shales, Millstone Grits, and 

 Lower Coal-measures, and, from my own observation, several of 

 Mr. Horwood's fourteen limestone plants grow frequently on other 

 formations. Specially may be mentioned Reseda luteula, Geranium 

 liicidum, and Hahenaria viridis. 



As an instance of the difficulty of defining with precision the soil- 

 distribution of plants, I may mention that I have for years regarded 

 Pteris aquilina as a limestone-hating plant. Here and there patches 

 of it occur in a limestone district, as in the Via Gellia, near Cromford, 

 but hitherto, with one exception, in all such instances the soil has 

 proved to be either an isolated patch of drift or a clay derived from 

 the decomposition of toadstone. The exception occurred last autumn 

 when, in the same neighbourhood, I observed Pterin aquilina growing 

 over an area of, perhaps, half an acre on an undoubted limestone 

 outcrop. Further examination revealed that the soil was derived 

 from the weathering of a dolomitized limestone, the chemical 

 composition of which seems to have reconciled the fern to its 



situation. 



Brook Cottage, 



Fkitchley, Derbyshire. 



February 10, 1914. 



H. C. Sargent, F.G.S. 



