128 Notices of Memoirs — Boulder-drift of North America. 



from north-east to south-west, with a gentle slope on the north- 

 west side and a very precipitous slope on the south-east. A broad 

 Silurian ridge, which separates this valley from Michigan, is covered 

 by a thick, coarse, stratified, water-worn gravel, which, at a distance 

 of twenty miles south-east, shades off into the Boulder-clay of 

 Illinois. The central part of this gravel deposit rises up into hills 

 800 feet above the level of Lake Michigan. The hills are often 

 sharp and conical, and interspersed Avith deep circular valleys with- 

 out outlets, called the Potash Kettles. The gravel runs south-west 

 about 200 miles, and is over 20 miles in width. In the northern 

 part it is coarse, with rounded stones up to more than a foot in 

 diameter, and yet thrown up into steep, round, lofty hills ; but 

 southwards the material becomes finer, and the hills lower, until they 

 shade off into the Illinois Prairies. On the north-west side of the 

 valley no gravel exists. The drift must have come from the north 

 in a vast sweep of water deep enough to cover gravel Mils 800 feet 

 high, and loith a velocity sufficient to tJirow coarse material up into 

 steep hills. The uninterrupted sweep of the water down the gradual 

 slope of the valley allowed no great deposits ; but the current 

 striking the opposite precipice must have been partly obstructed so 

 as to throw it into extraordinary and irregular commotion, here 

 piling up lofty gravel hills, and there leaving deep hollows, as it 

 swept across the broad ridge. The finer matter must have been 

 carried south, so as to settle as clay, and this accounts for the masses 

 of frozen gravel dropped by icebergs into the clay, and covered up 

 before the masses melted. A similar gravel-range flanks the south- 

 west border of the valley of the Georgian Bay, Lake Huron. 



The gravel hills, and nearly all the drifts of this region, are covered 

 with a thin stratum of orange-coloured loam, from a few inches to 

 several feet thick,' which follows all the undulations, excepting valleys 

 of erosion. It is free from boulders, and must have been deposited 

 in a quiet sea after the boulders ceased to arrive. 



Near the north source of the Western Drift the violence of the 

 aqueous action is evident. There is little or no drift on the north 

 slope of the Laurentian Hills. The whole country has been scratched 

 and pounded by the drift action, but the loose material has nearly all 

 been swept southwards. "Not even boulders could keep their 

 footing" in the current. The only exceptions to the sweeping action 

 are where abrupt declivities fronted the north so as to oppose the 

 current. There the finer matter is gone, but the boulders are piled 

 up in vast slopes, with their surfaces rammed together like a pave- 

 ment. Everywhere else there is smooth rock, only covered with a 

 little vegetable soil. Hundreds of miles of smooth rock run along 

 the northern slope of the Laurentian Hills. Facts testify to the 

 drift action for a thousand miles east and west along the Laurentian 

 Crest, and for an unknown distance north, having been too violent 

 to admit of deposition. South of the Crest the water was less 

 violent, and deposition commenced. 



1 I have noticed a similar stratum of loam covering the drifts of Yorkshire and 

 elsevyhere. — D. M. 



