58 DK. A. HOLMES ON THE PBE-CAMBBIAN [vol. lxxiv,. 



(3) Hornblende-Gneisses and Amphibolites. 



Attention has already been drawn to the association of horn- 

 blende-gneisses with crystalline limestones. The most complete 

 case studied in the field was that of the Ampwihi district (figs. 4 

 & 5, pp. 38-39), in which a gradual transition from biotite-gneiss 

 to hornblende-gneiss and garnetiferous amphibolite was traced as 

 the limestone was approached from the west. The biotite-gneisses 

 are of the coarsely-banded type, rich in microcline ; and, as the lime- 

 stone is approached, hornblende, accompanied by accessory sphene, 

 makes its appearance among the streaks of biotite. As hornblende 

 becomes more abundant, the rocks lose their banded and streaky 

 appearance, and show only a slight foliation due to the elongation 

 of some of the minerals in a common direction. Still nearer the 

 limestone the rocks become heavy and granulose without distinct 

 foliation, their dark surface, when freshly broken, presenting a 

 speckled appearance due to the presence of minute grains of felspar. 

 These rocks, which are neither schistose nor gneissose in structure, 

 are referred to as amphibolites. Garnet appears in the amphi- 

 bolites on the limestone side, and by increase of this mineral the 

 rocks merge into quartz-garnet-scapolite rock (JS T o. 19o, described 

 on p. 49). 



No. 189 may be taken as a characteristic example of hornblende-gneiss,, 

 showing foliation by the elongation of its minerals. In thin section it is 

 seen that the hornblende is slightly elongated parallel to the cleavages, 

 and that in many cases the cleavages and twin lamella? of the felspars 

 are approximately parallel to the same direction. The hornblende is chiefly 

 a brown variety, but it is very patchy in colour and passes through pale- 

 green to a nearly colourless tint. Minute apatites are present as inclusions, 

 but the chief accessory, around which the hornblende is frequently moulded, 

 is sphene. The rest of the rock, with the exception of a few sparse 

 shreds of biotite, is composed of rounded interstitial quartz and felspar, 

 the latter being beautifully fresh. Two thirds of the felspar is a soda- 

 lime variety approaching labradorite in composition, while the other third 

 is perthitic orthoclase or microcline. (PI. IX, fig. 2.) 



No. 194 represents the granular non-foliated type, distinguished as 

 amphibolite. The hornblende is of the same brown colour, but is more 

 abundant and less patchy than in No. 189 ; and instead of sphene inclusions, 

 titaniferous magnetite is present as the chief accessory mineral. Potash- 

 felspars and biotite have now entirely disappeared, and the soda-lime felspar 

 is labradorite, slightly more calcic than in 189. Another feature of interest 

 is the incoming of a very pale-pink garnet which generally forms a narrow 

 border between hornblende and felspar. The garnet is identical in appear- 

 ance with that of the quartz-garnet-scapolite rock which intervenes between 

 the amphibolite and the forsterite-marble. (PI. IX, fig. 3.) 



The mineral compositions of the chief rock-types, from biotite- 

 gneiss to the quartz-garnet-scapolite rock, have been measured by 

 the Rosiwal method and are set forth in Table VI (p. 59) with the 

 specific gravities in each case, as also the radium-content where 

 this has been determined. 



The gradual loss of quartz and the disappearance of the potash 

 minerals — orthoclase, microcline, and biotite, as the limestone is 

 approached, are offset by the steady increase of the lime-bearing 



