38 The Ottawa Naturalist. 



4. Locality. Knob or Fault Hill, west branch Hamilton River, 

 20 miles below old Fort Nascawpee on Lake Petitsikpow, (1) Labrador 



Peninsula. 



The specimen, according to Mr. A. P. Low", is from a dyke cutting 

 the ferruginous limestones and shales of Knob or Fault Hill, a promi- 

 nent topographical feature, as it rises rathet abruptly to the height of 

 350 feet above the surrounding country. The dyke occupies the sum- 

 mit of the hill, while 200 feet below come in the stratified rocks 

 through which it has been intruded. Neither the width of the dyke 

 nor the nature of its contact with the bedded rocks could be ascertained 

 owing to the accumulation of drift material, but it certainly cannot be 

 much less than 200 yards. 



Microscopically the hand specimen shows a medium textured dark 

 green almost black diabase containing occasional small and imperfect 

 phenocrysts of a light greenish grey plagioclase which has undergone 

 incipient " saussuritization." Under the microscope the rock is seen to 

 be composed of an aggregate of plagiochse, augite, serpentine, and 

 ilmenite. The augite is very fresh, Ins a light brownish red colour and 

 shows a marked pleochroism. In general i;s foim is allotriomorphic, 

 filling in the spaces between the felspar, but occasional individuals 

 exhibit sharp and perfect crystal boundaru I'he plagioclase occurs 



in inu:e or less elongated lath-shaped crystals which are often 

 somewhat stout and rounded thus producing a rather coarse ophitic 

 structure. Many of the small individuals a-e quite fre^i, but 

 the larger ones show considerable alteration to sericite and epidote. 

 The resulting " saussurite " is in no instance so abundantly developed 

 as to destroy the polysynthetic twinning stri;r. I'he large amount of 

 serpentine noticed in this rock has evidently resulted from the deconv 

 position of olivine originally present. The serpentinization of the olivine 

 is in every instance completed, and only the outline and structure of 

 the serpentine individuals serve to indicate the mineral from which it 

 has been derived. These occasionally show a network of fibrous ser- 

 pi ntine which was fust produced, the greenish fibres standing perpen- 

 dicular to the cracks along which they have been developed. Owing 



(1) Refer No. 4, \. p 28, Book II., Low, 21/6/94. 



