LANDSLIP IN PORTNEUP COUNTY, QUEBEC 487 



greatest length of 3,000 feet and an area of 86 acres. The floor of this depression 

 was formed by irregular mounds, pyramids, and blocks of clay, with trees, portions 

 of fences, and other debris and small pools of water here and there ; although it is 

 stated that very little water was seen during the actual movement of the mass. 

 The wrecks of trees coming from a wood-lot, part of which still remains near the 

 head of the crater, showed very clearly the direction of flow of the mass. The 

 channel of the Blanche below the orifice of the slip was entirely filled, and the 

 water spread from bank to bank of the valley amid mounds and blocks of clay and 

 debris that stood above it. 



Amount op Material involved 



The quantity of material which thus poured suddenly out into the Blanche 

 valley is approximately estimated at 93,654,000 cubic feet, with a total weight, ac- 

 cording to the specific gravity determined, of about 5,572,413 tons of 2,000 pounds. 



The slope of the original surface from the head of the collapsed area to the point 

 at which the road formerly passed, near the entrance to the narrow outlet, was, 

 according to barometric observations by Mr Chalmers, about 10 feet only. The 

 approximate diff'erence between the average level of the bottom from the head to 

 the present water level in the Blanche valley, according to tlie same authority, is 

 between 20 and 25 feet, while the slope of that part of the Blanche valley from the 

 orifice of the slip to the extremity of the flood of clay is not much more than 30 

 feet. 



Explanation of the Catastrophe 



The light slopes indicated by the above figures show that the mass of clay must 

 have simulated a liquid body when in motion. Mr Chalmers suggests that a lower 

 bed of the clay, in consequence of the impermeability of the subjacent boulder 

 clay, became exceptionallj' saturated, forming a sliding plane upon which the more 

 coherent overlying masses moved down. This would be in conformity with the 

 explanation usually (and probably in most cases correctly) given for landslips, and 

 it seems very likely that something of the kind may have been concerned in the 

 initiation of the slip here described where it began on the bank of the Blanche 

 valley. It appears to me, however, that the great and sudden discharge of clay in 

 this case should rather be attributed to the character of the water-saturated mass 

 as a whole, particularly as no evidence was found of any specially permeable or 

 iiuent bed and no underlying surface either of boulder clay or rock is anywhere 

 exposed. It will be noted that this landslip difl'ers very markedly in character 

 from the ordinary form, in which the subsidence occurs along an extended front. 



Three representative specimens of the clay, collected by Mr Chalmers while still in 

 a nearly saturated condition were submitted to a careful examination in the labora- 

 tory of the Survey under Doctor Hofl'mann's supervision. A mean of the results 

 obtained shows the specific gravity of the clay as received to have been 1.912, 

 equivalent to a weight of 119.5 pounds to the cubic foot. The clay as received was 

 found capable of absorbing a small additional amount of water, varying from 7.0 

 to 0.2 per cent by weight. Apart from the water, it consisted of 35.5 per cent of 

 argillaceous matter and 43.3 per cent of silt. When fully saturated it contained 

 on the average, which varied little in the three samples, 23.5 per cent of water by 

 weight or nearly 50 per cent by volume. 



