Section IV., 1885. [ 27 Trans. Roy. Soc. Canada 



III. — Geolofjy of CormvalUs or McNafs Island, Hali/a.r Harhnvr. 



Bij Rev. D. Honeyman, D.C.L. 



(Read May 28, 1SS5.) 



Thi.s, with Lawlor's Island, dividt\s the entrance to Halifax TIarbonr into two parts, 

 the main and eatstern passages. The only distinct geological formation of which it is con- 

 stituted is the Post-Pliocene or glacial drift. On its inner or north side is apparently a small 

 outcrop of Cambrian argillites, similar to those of Point Pleasant opposite. There is also a 

 rock off Lighthouse Point, which is seen at low water. To the east of this, on the elevated 

 ground, at the end of Lighthouse Beach, an outcrop of quartzite appears. These seem to 

 be continuations of the rocks of York Redoubt, on the west of the main entrance. The 

 elevation of the interior of the Island, 152, 145, 113 feet, indicates the existence of a solid 

 centre. 



The glacial drift is well exposed, in shore sections, along the west side of the Island. 

 Some of these have a height of twenty or thirty feet. They are distinguished at a distance 

 by their red colour. On the neck, between these and Thrum Cap, which can be tra- 

 versed only at low water, and even then by stepping stones and wading, are immense 

 boulders of c|uartzite in great number. These unquestionably were carried by glacial 

 action from Bedford Basin on the north. Among these is a large number of Archiean 

 boulders from the Cobequid Mountains, at a distance of seventy-five miles. These consist 

 of syeuitic gneisses and diorites containing magnetite, sometimes in considerable quantity, 

 as well as granites — some of which are hornblendic, porphyrites, etc. Associated with 

 these are boulders of basalt, having magnetite and olivine, and numei-ous ))Oulders of 

 amygdaloid, having amygdules of stilbite, heulandite and chalci^dony. These have ))een 

 transported from Blomidon or Partridge Islands from distances of sixty-three and sixty- 

 eight miles. Thrum Cap is wholly glacial drift, exi^oscd in red sections, which are washed 

 by the waves of the Atlantic. In these sections we collected, in the drift and /// xilii, 

 boulders similar to those on the neck. My associate, Col. Akers, R.E., directed my atten- 

 tion to a large mass of quartzite lying at the foot of the section. This was grooved and 

 scratched in a singular manner. I at once recognised it as part of the graving machine 

 that had grooved and striated the rocks of the Halifax Peninsula. The striation of Point 

 'Pleasant extended seaward touches Thrum Cap. The drift of this small island was doubt- 

 less a contiuiiation of that of McNab's Island. It was, in all probability, coterminous 

 (seaward) with Thrum Cap shoal, — the storms of the Atlantic having been the denuding 

 in both cases. Thrum Cap is of additional interest, as it is the advanced point of glacial 

 agencies transportation in Eastern Canada and America. 



