DALY : GEOLOGY OF THE NORTHEAST COAST OF LABRADOR. 213 



the almost universal covering of peat, to a grayish-white color highly 

 suggestive of weathered quartzite. The structure is linear-parallel and 

 is formed with extraordinary perfection. The gneiss is penetrated hy 

 dikes and more irregular intrusions of diorite which are probably related 

 as to age and origin with the trappean mass of Tub Island. 



Nowhere on this coast are trap-dikes so massive or so influential in 

 determining the topographic form of the country, as in the region about 

 Ice Tickle on the north side of Hamilton Inlet. Rodney Mundy Island 

 and Ice Tickle Island are each about three parts composed of medium 

 to coarse grained honi blende granitite, granitoid gneiss and fine-grained 

 gneisses. The remaining surface is occupied by a multitude of great 

 dikes, thick sheets, pods, or stock-like bodies of diabase which, on 

 account of their black color, contrast very strongly with the other crys- 

 tallines round about. The trap wherever examined is plainly intrusive. 

 No vesicular structure was to be found. Both contacts of a typical pod 

 showed the chilled and porphyritic zone characteristic of intrusion. A 

 fine example of the dikes is seen on the ridge at Indian Harbor. It has 

 parallel walls, is some two hundred feet in width, and is visible as a steep 

 black wall a quarter of a mile along the Tickle. Very commonly the 

 dikes are seen to swell out into thick trap-bodies three hundred to one 

 thousand feet in diameter, and again, a stock will project through the 

 gneisses without visible connection with the trap of the neighboring 

 hills. The average strike of the intrusive masses is N. E.-S. W. The 

 trap forms all the higher elevations, which thus assume the varying out- 

 lines of palisade, ridge, or dome, according to the shape of the intruded 

 rock-body. Between them the schists and granites, on account of their 

 inferior power to resist erosion, now underlie valleys that open sea- 

 ward on the deep bays and tickles of a "drowned " coast-line. One will 

 go far to discover so fine an example among wholly crystalline rocks, of 

 such control over land-forms — the control of differential resistance to 

 weathering. 



The same association of gneiss-valleys and trap-hills extends at least 

 ten miles to the westward of Ice Tickle. At Sloop Harbor, though still 

 plentiful, the dikes (here hornblende biotite diorite with accessory augite) 

 are too narrow to produce a great effect on the topography. Webeck 

 Island, a soilless, driftless, because wave-swept, granitic swell a half-mile 

 or more in length, affords a splendid exhibition of twenty-two huge 

 dikes cutting across the island in parallel fashion. On the mainland, 

 opposite Jigger Island near Webeck Island, an interesting group of dikes 

 in granitite was studied. One of these is a handsome biotite diorite 



