36 PEOr. T. G. BONIS-ET ON THE HUEOmAN SERIES 



variably with quartz (probably interstitial) *. The larger quartz 

 grains in the matrix are usuall}^ compound, consisting of several 

 granules rather polygonal in outline, and sometimes containing 

 between them tiny flakes of brown mica. 



The fragments in this altered conglomerate, east of Sudbury, are 

 interesting ; those examined in this rock are more or less rounded in 

 outline and light grey in colour. They consist of quartz, felspar, 

 and mica (brown and white), but the condition of these minerals is 

 peculiar. Broadly speaking, their association resembles that of a 

 moderately coarse granite, but the quartz on examination proves to 

 be not in single, or almost single, grains of fair size, but a mosaic, 

 like a honeycomb, of different granules, among which occur, very 

 sparsely, flakes of mica and earthy granules. The felspar has 

 almost lost not only all definite external form, but also to a great 

 extent its characteristic internal structure. Flakelets of white mica 

 and specks of quartz are developed in large patches over most parts 

 of the crystals, insulated portions only here and there remaining 

 comparatively untouched, and exhibiting sometimes the parallel 

 lamination of plagioclase, at others possibly the structure of micro- 

 cline, while others may be orthoclase ; nay, at times, even the larger 

 clusters of polygonal quartz appear to have been developed in the 

 heart of a large grain of felspar. The process of alteration seems to 

 be as follows : — Eirst a bit of the felspar assumes a ' dusty ' aspect ; 

 next some tiny flakes of mica and a granule of quartz segregate ; then 

 the latter enlarges, as it were pushing back the mica, which forms an 

 irregular ring round the grain (fig. 2). Then two or three grains 



Pig. 2. — Develojwient of Quartz and flakes of Mica in Fehj)ar Crystals. 

 (The mica is indicated by the outlined flakes, the quartz is 

 dotted. In the upper part of the diagram the larger mass of 

 quartz is beginning to show the " mosaic " structure.) X 27. 



grow together, generally expelling the intervening mica, so that at last 

 the felspar crystal is replaced by aggregated patches of quartz and of 

 microscopic mica. It is singular that while, from the general 

 appearance, one would be prepared to accept some of the quartz in 



*^Crystalline grains of blaci ircn-oxlde and of a dark granular mineral are 

 present. 



