32 The Ottawa Naturalist. 



gray almost opaque variety of sphene known as leucoxene. A strange 

 fact noticed, moreover, is that frequently the less altered phenocrysts 

 of Huronite occur in an exceedingly decomposed diabase as is the case 

 in the Pogamasing and Eastmain specimens, while the more highly 

 altered porphyritic individuals of this mineral are frequently developed 

 in a groundmass more or less remarkable for its freshness. This is 

 noticeably the case in the original specimen from the Drummond Island 

 boulder. 



Tne first stage in the decomposition or " siussuritization " of the 

 plagioclase shows a cloudiness due to the development of a dull, fine 

 grained, more or less opaque material, with a higher index of refraction 

 causing the granules to stand out in relief from the surrounding felspar. 

 In many cases, even in the thinnest sections, this is beyond the 

 highest power of the microscope to resolve into its component min- 

 eral or minerals. This is accompanied, or immediately followed, by the 

 development of sericite (hydrated muscovite) in small scales showing 

 characteristic brilliant interference colours. The cleavage planes and 

 fissures are seen to contain large scales and plates of this mineral, while 

 certain other cracks and fissures are filled with chlorite and serpentine 

 resulting from the decomposition of the bisilicates present. The smaller 

 granules now coalesce and form larger masses and individuals of zoisite 

 and epidote, while larger plates and scales of sericite are developed 

 and the original plagioclase is finally replaced by a comparatively coarse 

 grained aggregate consisting of zoisite, epidote, sericite, chlorite, calcite, 

 and felspar. Where the alteration has been extreme, as in the case of 

 the plagioclase originally piesent in the matrix of the Pogamasing 

 specimen, the lime is more or less completely removed, and the alkaline 

 portion of the plagioclase has crystallized into pure limpid grains of 

 albite which seldom show twinning striations and are accordingly fre- 

 quently mistaken for quartz with which they are often associated. 



The larger phenocrysts very frequently showed a marked difference 

 both in the degree and character of the alteration of their central and 

 peripheral portions. The zoisite and epidote were much more abund- 

 ant in the zone or belt immediately surrounding the crystals, while 

 muscovite is the prevailing decomposition product present in the cen- 



