34? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 4, 



his paper on the subject, that the deposit in which these boulders 

 occurred " was not produced by true glaciers, although intimately 

 connected with the phaenomena of ice." He also states, in the above- 

 quoted paper, that " the erratic blocks in Switzerland are always 

 angular," which is just what might have been expected if they were 

 transported by ice, whether upon glaciers or icebergs. The erratic 

 blocks in Scotland, on the other hand, are rounded, and we have 

 two problems to solve, — how have they been rounded? and how 

 have they been scratched? Before attempting to answer these 

 questions, it is necessary that we should be made acquainted with 

 all the circumstances under which these blocks are found ; we can 

 then compare the facts with proposed solutions, and by excluding 

 those which are inconsistent with well-established facts, be driven 

 at last to that solution which explains them all. 



Every person has seen pebbles ground on one side for the purpose 

 of exhibiting their polish or structure ; and the commonest observer 

 cannot fail to draw the following conclusions : — 



1. That the stone has undergone two different kinds of attrition 

 since it has been separated from its native rock, totally distinct and 

 unconnected with each other. 



2. The rounding process must have preceded the grinding one. 



3. The stone must have been at liberty to roll in the one case, 

 but must have been held fast in the other. 



4. The rounding has been caused by the action of water. 



The case of the boulders is precisely that of the pebble, and my 

 present object is to show how they have been held fast. I think it 

 right to observe, that the great mass of boulders at Bell's Hill, near 

 Glasgow, where M. Agassiz first observed this phaenomenon, and to 

 which he has since repeatedly alluded in his writings, although most 

 satisfactory on many points, threw no light upon this ; the boulders 

 were not in situ; a hill, composed of till, in which they were im- 

 bedded, had been levelled, and they were left in heaps. There was 

 therefore no means of knowing whether the cause, whatever it was, 

 acted in a certain and uniform direction, — -a point of great conse- 

 quence in this question. 



Some late observations, which I have had an opportunity of 

 making, convince me that the cause which has produced the furrows 

 upon the boulders is identical with that which has produced the 

 similar effect upon rocks in situ, and that they are posterior to the 

 deposition of the till, or at least to the deposition of the older 

 till or boulder clay, if it shall be established that there hiave been at 

 least two deposits of this nature. 



There are three positions in which boulders may be found in the 

 till : — They may be altogether buried in it ; they may be partially 

 buried in it, with their upper surface exposed ; or they may rest on 

 the surface. Rocks in situ may be covered with the till, or part of 

 their surface may be exposed. This distinction is an important one, 

 because if the striae on rocks occur below the till, they have probably 

 been caused by its deposition. If however they are only found on 

 the exposed surface, we may infer that they are posterior to it ; and 



