49 



In this pupei- it has been already assumed that at one tiuie there 

 Avas an extensive inland lake, which has gradually lessened till a few 

 small lakes and river stretches alone i-emain. The cause of the diminu- 

 tion is easily seen. The water has worn down the channels at what 

 were formerly rapids. When vast quantities of water rush annually 

 over i"ock surfaces the river beds must be lowered. Observation will 

 show that the rapids of the Upper Ottawa are natural barriers between 

 an upper and a lower water level, and between the rapids there are 

 ■even lakes or large riv^er expansions. With a given quantity of water, 

 a constant unit of time, and a homogeneous hardness of rock, the rate 

 of lowering of channel must be regular unless the seasons, rains and 

 freshets vary greatly. Even if all these were constant and regular at 

 present, we would be unable to locate the time in the past when the 

 mountains were 100 feet higher, or when any given river channel was 

 20 feet higher than now. 



By an examination of the rocks we can easily perceive that the 

 hardness varies. For example, the I'ock beds at the Narrows, a few 

 miles west of Pembroke, are very hard, say seven degrees out of a pos- 

 sible ten. But the channel rocks of the Allumette Bapids at the 

 opposite end of the lake from the Narrows, are only five degrees of 

 hardness. These rocks are a fine sandstone compacted with a bluish or 

 brown clay. The corroding action of the water tells readily upon the 

 clay-, and thus the sand grit is washed from the surface. 



What do we learn from this simple fact 1 This : The space between 

 the two rapids must 1)3 gradually drained of its water by the greater 

 corrosion of the lower rai)id?. This means that the present Upper 

 Allumette Lake is drying up, or draining off through the Allumette 

 Kapids becoming lower. 



This in turn means that the now navigable " Narrows" will in 

 time become too steep and shallow for steamboats to pass. But the 

 now unnavigable Allumette Bapids by that time will be navigable. 

 These are changes going forward to-day, and will be facts of the future. 



Again let us compare four parallel rapids, the Allumette, the Lost 

 Chenal, Beckett's Chenal and the " High Water Portage," an old chan. 

 nel of the river, but now completely dry even during high water. The 

 rocks of these four channels ai'e all of different degrees of hardness. 



