REPORT OF THE CHIEF ASTRONOMER 225 



SESSIONAL PAPER No. 25a 



Quartz 6-78 



Orthoclase. 2 78 



Albite 11-53 



Anorthite 30-86 



Diopside 21-07 



Hypersthene 1944 



Ilmenite 1-52 



Magnetite 4-41 



H 2 and CO, 1-28 



99-62 



Accordingly, in this method of classification, the type belongs to the pre- 

 sodic subrang of the percalcie rang, in the order, vaalare, of the salfemane 

 class. Tbe ratio of Q to F in the norm is very close to that which would place 

 the rock in the order, gallare. 



VARIATIONS FROM THE NORMAL. COMPOSITION. 



Variations from this gabbro type are very common in most of the sills. 

 These generally consist in an increase of quartz and biotite, along with the 

 appearance of orthoclase, which is crystallized either independently or in the 

 form of micrographic intergrowth with quartz. As these constituents increase 

 in amount, the hornblende seems to preserve its usual characters, but the 

 plagioclase shows a strong tendency toward assuming the zoned structure; the 

 cores average basic labradorite, Ab 1 An 2 , and the outermost shells average 

 andesine, near Ab 4 An 3 . When the quartz and micropegmatite become 

 especially abundant, the plagioclase averages acid andesine or basic oligoclase. 

 In several thin sections the plagioclase is seen to be mostly replaced by ortho- 

 clase and quartz, which, with the still dominant hornblendes, form the essential 

 substance of the rock. 



These changes in composition, indicating that the sill-rock has become 

 more acid, are always most notable along the contacts and especially along the 

 upper contacts. A good illustration of the acidification along the upper con- 

 tact occurs in a well exposed sill outcropping in the band that runs south from 

 the Boundary line at a point nine miles east of the Moyie river. This sill is 

 about 500 feet thick. A specimen (No. 1) taken twenty feet from the lower 

 contact is unusually rich in hornblende but bears much quartz and orthoclase 

 along with the subordinate essential, acid andesine. It carries no biotite nor 

 micropegmatite but orthoclase dominates over the plagioclase. Specimen No. 

 2, taken seventy -five feet from the lower contact, is a very similar rock in which 

 the plagioclase is an unzoned labradorite somewhat subordinate to the ortho- 

 clase in amount. Specimen No. 3, taken fifteen feet from the upper contact, 

 is gabbroid in look, though lighter in colour than either No. 1 or No. 2. It 

 is essentially composed of hornblende, quartz, orthoclase, and basic andesine, 

 named in the order of decreasing abundance. The accessories include micro- 

 pegmatite and much biotite, the latter in small, disseminated foils. The 

 essentials are all poikilitic with mutual interpenetrations and enclosures. The 

 structure is quite confused. 

 25a — vol. ii — 15 



