132 THE ICE AGE IN CANADA. 



increasing in size, and. actually becomes a miniature berg, 

 containing some thousands of cubic feet of ice and mud, 

 and still retaining a buoyancy which will enable it, after 

 a thaw, during high spring tides, to break away with a 

 load of dSbris, and carry it either out to sea or up the 

 estuary, and, if it should chance to be stranded again, it 

 will probably leave a portion of its burden, provided it 

 has not melted off during its voyage with the tide. But 

 there can be no doubt that some of the attached sand, 

 mud, or shingle is melted off during the journey of the 

 block or miniature berg, and drops into the bed of the 

 river or estuary. In reality, these ice-cakes, when in 

 motion, are perpetually strewing the bottom with trans- 

 ported material and bringing a portion from one place to 

 another, during about five hours of the flood, and carrying 

 part of it back again, during five hours of ebb, to the 

 limits of tlie backward and forward tidal range of each 

 particular ice-cake. But when they accumulate in an 

 eddy, they become powerful carriers and depositors of 

 detritus, and if artificial obstructions be introduced so as 

 to form an eddy in the usual course of the ice-stream, the 

 accumulation must necessarily be very rapid." 



6.— CONTINENTAL ELEVATION AND DEPRESSION. 



6. Before leaving this summary of causes, it is necessary 

 to make a few general statements respecting elevation 

 and depression. The first and most important is that, 

 from the great Pliocene elevation onward, subsidence 

 and re-elevation were always in progress. At each stage 

 of these there must have been corresponding geogra- 

 phical conditions and varying facilities for distribution 

 of travelled detritus. In regard, therefore, to the causes 

 of any particular deposit, one of the most important 



