58 Natural History. 



the vasty deep purely for the convenience of a fluviatlle Crus- 

 tacean, however interesting it may be.* 



Any alteration in elevation or in the sculpture of the land 

 relief involves changes in the whole series of plants and 

 animals which naturally depend upon it. Thus, for example, 

 our district at Dumfries has passed through not one, but 

 several periods of intense volcanic activity. It has enjoyed 

 ages of repose either beneath deep sea or in the quiet waters 

 of an estuary. The conditions at Canonbie and Sanquhar 

 must have resembled at one time a West African Mangrove 

 swamp ; at another Nithsdale seems to have been as dry and 

 arid as the Hammada el Homra of the Sahara. These few 

 instances of change in our theories regarding an ever-changing 

 world show how diiferent is our standpoint to-day from that 

 even of ten years ago. 



But it is the detailed work of the last fifty years that is 

 perhaps the most astonishing part of it. During the last fifty 

 years probably every single rock and mineral in the British 

 Islands has been sliced in thin sections and examined with 

 microscope and polariscope, and has so revealed the mysteries 

 of its formation. 



Every stage in the advance and retreat of glacial con- 

 ditions during the last Ice Age, or series of Ice Ages, has 

 been carefully studied, and with the result that the outline, 

 first sketched by the genius of James Geikie, is now almost 

 a history of events with dates. 



Glaciers from our Galloway hills have been traced to the 

 Irish Sea, where they had to take part in that bewildering 

 struggle of ice-sheets from many directions, from Ayrshire 

 and Argyllshire, as well as with those from Ireland and Wales. 

 As regards our own district, granite fragments from Criffel, 

 Dalbeattie, and other Galloway hills have been carefully 

 tracked, and prove that the Galloway ice passed up the Eden 

 and across Teesdale into Yorkshire. 



On the East coast, the Scotch ice was also forced into 

 \'orkshire by the pressure of the Scandinavian ice-sheet. 

 Rock fragments found in Yorkshire or dredged from the 



* Hedley, Proc L'mn. Soc. of Londun, October, 1912. 



