DILUVIUM. DRIFT AND ERRATIC BLOCKS. 



763 



blocks of granite, from 15 to 25 feet in diameter, on the Coteau des Prairies, to the 

 west of Lake Superior, several hundred miles from the nearest rock of that kind in place, 

 which lies northward. 



3. Mr. Darwin affirms, that in the intertropical parts of America, Africa, and Asia, 

 erratic blocks have never been observed, nor at the Cape of Good Hope, nor in Australia. 

 Though this complete limitation of them to the colder regions of the globe may be 

 deemed an unsettled point, it is certain that such phenomena are far less frequent and 

 extensive in equatorial countries. Beyond 41 of south latitude in South America, the 

 appearance of drift is common, upon the vast plains of Patagonia and Chili. 



4. Rocks in situ, or in place, very frequently exhibit a marking with fine linear strife 

 or scratches, larger grooves, and even furrows, with a smooth and polished aspect of the 

 surface, like that which the rapid passage of heavy masses over them would produce. 

 Transatlantic facts of this description have been carefully collected by Professors 

 Hitchcock and Rogers, from whom a few particulars may be introduced. In various 

 parts of Massachusetts the strife are very obvious and distinct, frequent on the hard 

 sienitic rocks, though often these are merely smoothed, and sometimes polished. They 

 are visible on the gneiss at the top of the Wachusett mountain, the highest in the 

 eastern part of the state, 2000 feet above the ocean. The precipitous hills and the 

 lower grounds of the Connecticut river valley are covered with them, their direction 

 being north and south, inclining a few points east of south, and west of north. Mount 

 Everett, 2600 feet above the sea level, has been worn over its whole surface, and the 

 striae are very visible in many places, though so long exposed naked to atmospheric 

 and decomposing agencies. Mount Monadnoc, 3250 feet high, little else than a naked 

 mass of mica-slate, of peculiar character, and almost destitute of stratification, has been 

 from top to bottom scarified and scratched, the striae running about north-west and 

 south-east. Similar markings appear on the summits of all the Appalachian chain 

 in Pennsylvania, which observe the same direction, and particularly, around the Wyoming 

 valley the tops of the mountains are covered with parallel strice. It is impossible, says 

 one of the authorities named, to stand upon these lofty and precipitous ridges, and witness 

 this phenomenon, without being struck with the great power and extent of the agency that 

 has thus left its traces upon some of the most elevated spots in New England. The cut 



on the next page will con 

 vey an idea of the effect in 

 question, a surface of rock 

 smoothed and striated. The 

 same appearances have 

 been noticed in North 

 Wales, on Corstorphine 

 hill near Edinburgh, and 

 other parts of Scotland, in 

 Sweden and North Russia. 

 In some instances the tops 

 of ledges of rock appear to 

 have been crushed, and 

 bent obliquely by the pres 

 sure upon them of an enor- 



Broken Ledges of Slate. moug loa( J in movemen t. 



In the annexed view, taken from a quarry of clay-slate in the state of Vermont, the 

 perpendicular strata appear broken, and partially overturned, towards the summit. 



The occurrence of drift, and of erratic blocks, constitutes one of the most complicated 



