322 Homer's Geological Address. 



of pebbles of granite and quartz, the hardest of which are cut 

 through as clean as the softer argillaceous cement. The erod- 

 ing tool has acted to the length of forty-five metres (about fifty 

 yards), on a surface inclined from 45° to 50°, and with a breadth 

 of from four to five metres (thirteen to sixteen feet). But my 

 limits oblige me to refer you to the memoir itself, and to the 

 report of the discussion to which it gave rise, for many most 

 interesting facts, and some important views as to the causes of 

 these remarkable phenomena.* For the same reason, I can 

 only very briefly allude to the descriptions contained in several 

 parts of Mr Ly ell's " Travels," of the boulder formation, the 

 erratic blocks, and the furrowed surfaces, that are met with 

 over a great part of the northern regions of North America, 

 presenting many features identical with those of Northern Eu- 

 rope. 



In Europe the boulder formation has not been traced farther 

 south than 52° north latitude, but a similar kind of detritus, 

 sand, clay, gravel, and rounded blocks of great size, cover a 

 considerable extent of country in the neighbourhood of Boston, 

 which is ten degrees farther south, or about the latitude of 

 Valencia in Spain. It is not found within the range of the 

 Alleghany mountains ; but blocks again appear on their western 

 side, near the Ohio river, in latitude 40°, and some scattered 

 blocks have reached Kentucky, the northern boundary of that 

 state, in lat. 38| °. How far a boulder formation, erratic blocks, 

 and furrowed rocks, extend beyond the valley of the St Law- 

 rence, we have yet to learn ; but the scanty information we do 

 possess leads us to infer, that they exist on the shore of the 

 Arctic Sea. 



Near Boston the boulder formation has been pierced to a 

 depth of more than 200 feet without the solid rock having been 

 reached ; and although mainly composed of the materials of 

 neighbouring rocks, huge rounded blocks brought from a great 

 distance rest upon them or are buried in them. Here, as in 

 Russia and Denmark, we have a boulder formation composed 

 of materials that have not been far travelled, intermixed in 

 some degree with, but more frequently covered by, that of 



* Bulletin de la Soc. G60I. de France, tome iii., p. 65. 



