364 Transactions. — Geology. 



Enstatite Diorite. 



Dusky Sound. — A rather pale-grey, finely-grained rock, not 

 showing foliation. Under a lens it is finely speckled with 

 black and white. Composed of plagioclase and hypersthene, 

 the latter occupying about one-quarter of the rock. The feld- 

 spars go up to O'lin. in length, but show pressure-granulation. 

 The hypersthene is mostly in uncleaved small grains enclosed 

 in feldspar, but occasionally it is in allotriomorphic crystals up 

 to 0-04:in. in length. It is strongly pleochroic, and not schil- 

 lerised. 



From the head of Wet Jacket Arm. 



Enstatite Gabbro. 



Milford Sonnd. — A dark, almost black, rock, composed of 

 hornblende and feldspar, the latter occupying less than half the 

 rock. The feldspars are chiefly plagioclase, but some seem to 

 be microcline. In addition to the hornblende there is a little 

 enstatite, which is nearly colourless, and slightly pleochroic. 

 There is also a little muscovite. No pressure-granulation. 



From Harrison's Cove. A similar rock is found in many 

 places in Milford Sound, and has been called hornblende- 

 schist. 



Age of the Bocks. 



The large size of the elements of these rocks shows that 

 they are of plutonic eruptive origin, while the pressure-granu- 

 lation observable in many of them shows that the foliated 

 structure is of secondary origin. It will be noticed that the 

 size of the elements is approximately the same all through, 

 and the coarseness or fineness of the grain depends upon the 

 amount of segregation the minerals have undergone. 



They have been considered as Arch^an on account of litho- 

 logical structure, and not from any stratigraphical evidence. 

 Consequently, this opinion cannot now be sustained. On the 

 contrary, the absence of contortion, and the almost universal 

 westerly dip of the foliation planes, is strong evidence that 

 they are not Archaean. What their age may be it is impossible 

 to say at present. Sir James Hector says that " wrapping 

 round these crystalline strata, and sometimes rising to an 

 altitude of 5,000ft. on its surface, is a series of hornblende- 

 schists, soft micaceous and amphibolic gneiss, clay-slate and 

 quartzite, associated with feldstone dykes, serpentine, and 

 granular limestone;" and he believes these "to be meta- 

 morphic rocks of not very ancient date — probably of Devonian 

 age."''' Some of these are no doubt sedimentary rocks, 

 altered by contact with the eruptive diorites, and the diorites 



* " Outlines of the Geology of New Zealand," Wellington, 1886, p. 51 ; 

 and Quart. Jour. Geol. Soc. of Lond., vol. xxi. (1865), p. 124. 



