REPORT OF THE CHIEF ASTRONOMER 339 



SESSIONAL PAPER No. 25a 



crowded with inclusions of the older ones, yet in other instances, types of 

 quite diverse appearance seem to pass gradually into one another. The 

 different varieties in some cases occupy large areas to the exclusion of 

 other types, while in other places they appear as dike-like or quite 

 irregular bodies within one another. 



' It was not thought profitable to attempt to map separately the differ- 

 ent varieties of monzonite, especially as they are all believed to be closely 

 related in origin and composition and to have been nearly contemporane- 

 ous. As regards the relative ages of the different varieties it would seem 

 that, in general, the coarser types are younger than the finer and the more 

 feldspathic and lighter coloured varieties are younger than, the darker. 



' The coarsest type of monzonite and the one most readily separated 

 in the field from the other varieties, occupies a large area stretching from 

 the shaft of the Great Western mine to near the head-works of the LeRoi. 

 Smaller areas of a similar type are common on the south face of Monte 

 Cristo mountain and also along the southwestern border of the monzonite 

 body. This coarse type is usually of a dark colour and consists largely of 

 dark, nearly black prisms of pyroxene or secondary hornblende, flakes of 

 biotite, and a light coloured feldspar, that gives the appearance of lying 

 between the other constituents. In many instances the augite and horn- 

 blende form the bulk of the rock, occurring in both large and small, often 

 ragged, prismatic forms frequently varying between one quarter and one 

 half an inch in length. The dark brown biotite, though never as plentiful 

 as the other dark silicates, is abundant and forms large irregular flakes. 

 The feldspars are usually white or slightly greenish in colour and appear 

 to lie between the prisms of augite and hornblende, though when seen in 

 thin sections they often have sharply rectangular outlines; they are chiefly 

 labradorite, with interstitial orthoclase in more subordinate amount. 



' This type of monzonite frequently shows local variations along bands 

 where the feldspars sometimes almost dissappear, the rock then assumes a 

 greenish black colour and is composed nearly altogether of coarsely 

 crystalline hornblende and pyroxene with much biotite. Sometimes this 

 type seems to end abruptly against the surrounding varieties of more 

 normal monzonite, while at other times it presents transitional forms in 

 which the feldspars increase in amount while the dark coloured constitu- 

 ents decrease in both size and quantity; the remaining larger individuals 

 of pyroxene or hornblende may then give a porphyritic aspect to the rock. 

 Along the southern border of the monzonite body this type or a related 

 one, holds large poikilitic biotite flakes measuring a quarter of an inch or 

 more in diameter and there cuts and holds inclusions of a finer grained 

 variety of monzonite. 



' The remaining varieties of monzonite present characters that often 



remain fairly constant over considerable areas and while examples from 



; different localities may appear quite dissimilar, yet they possess certain 



features in common and it would he quite possible to select a series of 



25a— vol. ii— 22£ 



