part 2] OF souTHEEisr eyee peis'insula. 105 



The source of the prehnite is presumably the plagioclase : 



CaAl,Si,0, -|-Ca(OH)2 + SiO^ = H.Ca^Al.Si.^Oi,. 



Calcium hydroxide could be derived from the pyroxene-amphi- 

 bole conversion, but the source of the silica must be external, as 

 quai-tz is associated v^ith the prehnite itself. 



(/>) The Pyroxene-bearing Types: the Pyroxene-Granulites. 



These rocks are closel}^ related to the amphibolites. They occur 

 as bands in the gneisses of Sleaford and Fisher}- Bay, and are 

 represented in the dark bands which intersect the Boston -Island 

 acid gneisses and granites. They can usually be distinguished in 

 the field by their lighter coloration, as compared with the trne 

 amphibolites ; although, if the content of hornblende is large, this 

 is not readily possible. A number of these types will be de- 

 , scribed briefly. 



(1) Pyroxene -Amphibolite from Sleaford Bay. 



The constituents as seen under the microscope are hypersthene, 

 augite, hornblende, plagioclase, orthoclase, magnetite, apatite, and 

 pyrites. The two pyroxenes are seen in parallel intergrowth. In 

 longitudinal section the inner pyroxene has straight extinction 

 and the properties of hypersthene, and the outer zone has the pro- 

 perties of a monoclinic member — diopside or augite. In some cases 

 this position is reversed, and the monoclinic member occupies the 

 inner zone. The monoclinic pyroxene has developed a prominent 

 diallagic lamination parallel to (100). A development of uralite 

 at the edges of the pyroxene-grains is to be observed. 



The hornblende has a habit such as to indicate it as a primary 

 crystallization, and not replacing pyroxene. 



Twinning in the plagioclase is not characteristic, and its 

 composition points to calcic andesine. The potash-felspar is 

 readil}^ distinguished by its low refractive index, and for the 

 greater part occurs in interstitial bands. 



The order of abundance of the minerals in the rock is : 

 plagioclase, augite and hj^persthene, orthoclase, hornblende and 

 magnetite, apatite, pyrites. 



The composition in general is a mineralogical association of 

 plagioclase, pyroxenes (monoclinic and rhombic), hornblende, and 

 the accessories magnetite and apatite. In certain types additional 

 minerals enter as accessories : orthoclase, biotite, garnet, quartz, 

 pyrites, and zircon. The structure of the rocks is often quite 

 granulitic, none of the constituents possessing an}'' well-dehned 

 crystal-outline, and closely resembles that of the well-known 

 granulites of Saxony. This granulitic habit is, however, not a 

 constant feature of the rocks of this series, and in a number of 

 the Sleaford types the minerals have a distinct gabbroid structure. 

 The tabular character of the plagioclase is then a marked feature, 

 and the twinning lamellation is universal. 



