THE SUCCESSION OF DEPOSITS. 43 



when, as is not infrequent, we have two sets nearly at 

 right angles to each other, in the same locality. Hence it 

 becomes an important question to ascertain the relative 

 ages of the striation, and also the direction in which the 

 abrading force moved. 



Taking the valley of the St. Lawrence in the first 

 instance, the crag-and-tail forms of the isolated hills of 

 trap, like the Montreal mountain, with abrupt escarp- 

 ments to the north-east and slopes of debris to the south- 

 west, the quantity of boulders carried from them far to 

 the south-west, and the prevailing striation in the same 

 direction, all point to a general movement of detritus up 

 the St. Lawrence valley to the south-west. Further, in 

 some cases the striae themselves show the direction of the 

 abrading force. For example, in a fine exposure recently 

 made at the Mile-end quarries, near Montreal, the polished 

 and grooved surface of the limestone shows four sets of 

 striae. The principal ones have the direction of S. 68 W. 

 and S. 60 W. respectively, and the second of these sets 

 is the stronger and coarser, and sometimes obliterates the 

 first. The two other sets are comparatively few and 

 feeble striae, one set running nearly 1ST. and S., and the 

 other N.W. and S.E. These last are probably newer than 

 the two first sets. Now, with regard to the direction of 

 the principal sets of striae, this at the locality in question 

 was rendered very manifest by the occurrence of certain 

 trap dykes crossing the limestone at right angles to the 

 striae. The force, whatever it was, had impinged on these 

 dykes from the N.E., and their S.W. side had protected 

 the softer limestone. The locality is to the N.E. of the 

 mass of trap constituting the Montreal mountain, and the 

 movement must have been up the St. Lawrence valley 

 from the N.E., and toward the mountain, but at this 



