Mb. HOPKINS, ON THE TRANSPORT OF ERRATIC BLOCKS. 239 



points then become subject to the greatest attrition, and the surface afterwards assumes that 

 form and polish which distinguish a water-worn boulder. I do not recollect, however, to have 

 observed on any beach instances of this perfect rounding and polishing except in pebbles, much 

 too small to afford any explanation of the cases of many of the erratic blocks which have been 

 subjected to some similar and equally effective process of that kind. Moreover, should the 

 efficiency of this cause be allowed, it must be recollected that the sphere of its operation is 

 limited to the comparatively small area over which the waves break, for it is there alone that 

 they can exert any effective power. How then shall we thus account for the water-worn appear- 

 ance of innumerable blocks existing in the detritus spread out over a wide area, or in cases 

 where the transported materials exist in layers of great thickness .'' If it should be contended 

 that the water-worn appearance may be due to the other cause above alluded to — the action 

 of water remote from shallow coasts — it must be replied, that that force which is capable of 

 rolling a block is unquestionably sufficient to transport it, and therefore, that the solution does, in 

 fact, admit the existence of transporting currents. 



There is also another important point to be remarked with respect to the transport by ice, 

 whether on land or by water — it affords no reason why the transported blocks should diminish 

 in size, and become more generally rounded and polished, the more distant they are from 

 their original localities. Such would necessarily be the consequences of transport by currents, 

 but it must be a matter of indifference whether a block has been floated on an iceberg or 

 carried by a glacier one mile or one hundred miles, so far as regards the form and dimensions 

 of the block when ultimately deposited by the ice which conveyed it. If the great majority of 

 the blocks transported from a given locality be rounded and polished, there is a strong presumption 

 that water has been the transporting agent ; if, moreover, the blocks do not exceed a weight 

 of a few tons, the probability of that mode of transport is increased ; and, finally, if we find 

 that the magnitude of the blocks generally diminishes as their distance from their original 

 site increases, till at length they degenerate into rounded pebbles, the previous probability 

 appears to me to approximate as nearly to certainty as we can reasonably expect. 



On the other hand, when erratic blocks are extremely large, the presumption is in favour of 

 their having been transported by ice ; and if, moreover, they retain sharp angular points and 

 edges on their apparently unworn surfaces, and their magnitude bears no relation to the distance 

 of transport, we may confidently conclude that the transporting agent has been ice, assuming 

 always that the transport is attributable to one of the causes we have mentioned. 



The main distinction between the cases of transport by glaciers and by floating ice, must be 

 sought for, I conceive, in the distance which the blocks have travelled, and the nature of the 

 surface over which the transport has taken place, and not in the character of the blocks them- 

 selves. If the motion of glaciers be due to gravity, as I have endeavoured to shew in a recent 

 memoir, it would be an absurdity to attribute to their agency the transport of the blocks dis- 

 seminated over the extensive flat plains of northern Germany and Russia. In such cases I 

 should not hesitate to refer the removal of large angular blocks to the agency of floating ice. On 

 the other hand, the transport of numerous blocks on the flanks of the Alpine chain can hardly 

 be referred to any agency but that of glaciers of greater extent than those now existing. In 

 other cases the transport may have been effected by a combination of these means. Blocks may 

 have been brought down by glaciers from the mountains, and then floated on icebergs to distant 

 localities. This process has been recently observed, on a magnificent scale, in a high northern 

 latitude, and appears to me the simplest mode of accounting, in certain cases, for the transport 

 of blocks now far above the level of the sea. If Switzerland were depressed iGOO or 17OO feet 

 below its present level, the enormous angular block of Pierre a hot above Ncuchatel would be 

 on the margin of an arm of the sea, occupying the present valley of Switzerland, while on the 

 opposite margin there would bo rocks bearing the strongest marks of glacial action. Under 

 this hypothesis, and without assuming any material change in the general configuration of the 



