On Some Dykes Containing Huronite. 31 



zoisite, epidote, sericite and chlorite at the expense of the original 

 felspar. Some of the phenocrysts show a more or less perfect cleavage 

 which is noticeably the case in the Eastmain specimen, although in the 

 more hightly altered samples, as those from the vicinity of Missinaibi, 

 little or none can be seen. Occasionally, crystals show macroscopically 

 the lamellation due to polysynthetic twinning, as in some of those in 

 the Murphy Lake diabase, but as a general rule these lamellae are 

 either absent altogether or so faint that they cannot be detected. The 

 mineral is sublranslucent, varies in lustre from pearly to waxy accord- 

 ing to degree of alteration. The hardness varies from 5^ to 6, fusibility 

 about 5, and the specific gravity, according to Mr. R. A. A. Johnston, 

 of the Geological Survey of Canada, varies from 2725 in the East- 

 main specimen to 2 935 in those from Missinaibi. The specific 

 gravity, as would be expected, shows an increase in proportion to the 

 the alteration. The microscopic examination in general reveals the 

 fact that in every case the so-called " Huronite " is really a plagioclase 

 near the basic end of the series which has undergone more or less 

 complete "saussuritization." In most instances the development of zoisite 

 epidote, sericite, chlorite, etc., at the expense of the original felspar has 

 been so abundant as to leave only traces of the original twinning 

 lammellse and occasionally to destroy all evidence of this structure. 

 Specimens may be obtained from the large number of slides examined, 

 showing a complete gradation of this decomposition from the pure 

 glassy plagioclase (labradorite) composing many of the phenocrysts con- 

 tained in the diabase from Temagami Lake to the completed Saussurite 

 or Huronite in the porphyritic individuals of the Missinaibi rock. The 

 matrix in which those phenocrysts are embedded is in general a typical 

 diabase of dark greenish or greyish colour which likewise shows a wide 

 difference in degree of alteration under the microscope. The speci- 

 mens from Bear Island, Lake Temagami, show a very typical and fresh 

 olivine-diabase. With the exception of some of the crystals of olivine, 

 the rock is remarkably free from decomposition, while in the finer 

 grained portion of the rock from Missinaibi all the component minerals 

 have undergone great alteration. The plagioclase is more or less 

 completely " saussuritized," the augite originally present wholly con- 

 verted to hornblende (uralite) and the ilmenite replaced by the dull 



