On Metallic Minerals. 417 



renders more probable tiie following explanation of the 

 cause ol the magnetic attraction in tiie neighbourhood of 

 L'Acadie, viz. that it is owing to the reappearance of the 

 same trap on the surface of the ground wliicli, as in the 

 instance of the Montreal mountain, is known to be 

 very magnetic. 



'1 his mountain has a remarkable action upon the mag- 

 netic needle. Wijile surveying on its summit some years 

 ago, I observed a variation of 4** in a distance of one 

 hundred and seventy feet. At first some error was suspected 

 in the operation, but by frequent trials and with dilTerent 

 instruments, the existence of this phenomenon was placed 

 bevond all doubt. 



It was a somewhat singular coincidence, that while 

 em[)Ioyed ascertaining the fact, Mr. Sliand, the overseer of 

 works in the Engineer Department, who Ijad been employed 

 running a line by compass on a another part of the moun- 

 tain, joined me to report a much greater deviation of the 

 compass in his case than in mine, we were afterwards 

 obliged to work without any reference to a compass, which 

 is unquestionably the best plan at all times. 



Lieut. Luxmoore, R. E. while measuring a base on the 

 ice opposite Montreal, a few winters ago, found a variation 

 of W in about two thousand five hundred yards, probably 

 the effect of the same cause, though weakened by distance. 



It is the character of the trap rocks, like the Montreal 

 mountain, to effect the compass. The basaltic range iu 

 Ireland, called the " (iiant's Causeway," does so in a 

 remarkable degree. This phenomenon is not owing to 

 tbe«c rocks containing beds or veins of magnetic iron, but 

 to the iron entering into the con)position of the rock, being 

 magnetic ; and a* those rocks in many cases are decidedly 



