MINERALOGY AND PETROLOGY. 983 



Petrographic and petrologic.] 



greenstone eastward till it comes into contact with the gabbro. A similar transition 

 takes place at the northeastward extremities of the narrow lakes through which the 

 Kawishiwi river flows, sections 15 and 16, T. 63-9 W. (Nos. 982, 983, 984), and also in 

 the greenstone hills northward from Chub lake. Indeed it would be entirely correct 

 and safe to state that it takes place wherever the gabbro in its recognized form 

 comes. onto or near the Keewatin greenstone. 



Again, in some instances, the greenstone is conglomeratic, the pebbles being of 

 different phases of greenstone, of granite and of slate. In such cases, where the 

 original rock is basic enough, the complete transition is easily traceable through 

 muscovadite to the normal gabbro, the pebbly forms being visible in all the rocks. 

 This is notably the case at the point often referred to south of Disappointment lake, 

 described in more detail in volume iv, page 303. Note, also, the pebbly aspect of 

 the granulitic gabbro shown in plate MM, figs. 5 and 6, vol. iv. " This is a remarkable 

 rock, as it resembles muscovadyte, which we suppose to be the result of a change in 

 sedimentary rocks; it is a remarkable circumstance, also, that so far south within 

 the gabbro area so much of this rock is found. It is heavily jointed, lies nearly hori- 

 zontal and slides in sheets into the lake, toward the southeast, the sheets being from 

 one-half inch to six inches thick. Small nodules weather out on the surfaces, and 

 some larger, harder patches also appear, resembling some seen in the changed gray- 

 wackes on Gabemichigama lake. This rock prevails about the shores of Muscovado 

 lake, on the shores of the north half of Bashitanaqueb lake, and just north of the 

 latter it forms some high hills."* This is rock Nos. 1784 and 1785. It is evident that 

 here is a large area of the original greenstone, well within the gabbro, which has not 

 suffered a complete conversion to typical gabbro, and that the original greenstone 

 was conglomeratic and similar to that at Disappointment lake. 



2. Petrographic and petrologic. The petrographic facts have been for the most 

 part enumerated in an earlier part of this chapter in connection with the descrip- 

 tion of the minerals that compose these rocks. There is nothing more evident in 

 reading the special descriptions of the rocks referred to in Part II, than the "air of 

 family " that binds the gabbro, the muscovadyte, the noryte, or granulitic gabbro, in 

 one genetic class. The ophitic structure of diabase is found to occur in the same 

 rock in conjunction with the granulitic. Sometimes, in the granulitic gabbro, or 

 muscovadyte, the pyroxene is simple augite and sometimes diallage. Frequently 

 hypersthene is abundant and sometimes it is wanting. Sometimes hypersthene is 

 twinned with a monoclinic pyroxene and sometimes it poikilitically embraces labra- 

 dorite, quartz and globular hypersthenes. Olivine, and even magnetite (No. 695) 

 and biotite sometimes prevail over all the other minerals, in the last making a mica 



*N. H. WINCHKLL. Twenty-first Annual Report, p. 160. 



