Mr Hugh Miller on Bouldcr-Glaciation. 173 



(1.) Of what practical value boulder-glaciation will prove 

 in drift-covered districts generally, especially among the im- 

 perfectly ^vrought-up drifts of southern England, remains to 

 be seen. Dr Croll has proposed to determine the movements 

 of the ice in these regions from the direction assumed by 

 contortions in the drift.^ This method by means of pave- 

 ment-boulders seems more workable, and has at least the 

 advantage of having been tested. 



(2.) The extent to which the successive surfaces of the 

 boulder-clay have registered the changes in ice-flow during 

 its formation can best be illustrated by a concrete example. 

 Fig. 6 represents two boulder-clays differentiated by (among 

 other things) the glaciation of the boulders they contain in 

 directions at riglit angles to one another. The cross-liatch- 

 ing on rock-surfaces, which has hitherto been brought in as 

 evidence, doubtless register one or two of the earlier changes 

 of ice-movement. But, except where it can be supposed 

 that a cover of boulder-clay has been laid down and stripped 

 off again in the interval, we have been really left to general 

 evidence as to the variations that supervened during the 

 stages of the deposit. Questions respecting the intercrossing 

 of erratics also remain as yet questions of general evidence. 



If the till were an evenly deposited formation, it might be 

 expected that all the changes of the ice-currents should be 

 registered in its successive layers. But the till, of course, is 

 an irregular formation. Every renewal of impetus or change 

 of direction probably caused the ice to attack its own deposits 

 at changed angles or with renewed force. No one can study 

 the till minutely without seeing that erosion and reconstruc- 

 tion were in progress simultaneously. 



In the neighbourhood of Edinburgh the direction of glacia- 

 tion seems to have remained pretty constant throughout. The 

 same may also be said, though to a less extent, and with 

 remarkable exceptions, of the county of Northumberland. But 

 in the west of the county — in that large tract of low ground 

 that borders the Solway — it appears to have been otherwise. 

 It is probable that the Solway Firth may be viewed as a 



1 Climate and Time, p. 462. 



