Vol. 70.] OF THE COTJNTET AROUXD HUJfTLY. 275 



but is likewise poecilitic with the olivines. Here, too, are seen 

 scales of a chloritic mineral and laths of actinolite, which are 

 especially well|represented in the reaction -borders. 



(c) The rocks of the central intrusion. — The dulled 

 margin of this intrusion betrays its banded nature bv the ease with 

 which it splits along certain planes. It is fine-grained and 

 yellowish, showing felspar, rare flakes of biotite, and another ferro- 

 naagnesian mineral, which microscopic examination proves to be 

 a weakly-pleochroic hypersthene. The mineral banding is now 

 seen to be due to the alignment of the minerals, their directions 

 of greatest length being parallel. The felspars are fresh, and the 

 symmetrical extinction -angles would indicate that they are not 

 more acid than a medium labradorite. Biotite is not abundant, 

 but occurs with the pyroxene and grains of magnetite. A slight 

 variation is afforded by a type near Broomhillock, where the 

 hypersthene is often surrounded by a shell of faintly-pleochroic 

 yellowish-brown hornblende. Here, too, the foliation is less evident. 



The, second type, much coarser and possessing a granitic texture, 

 shows a yellowish felspar and glistening biotite. In section 

 the mineral content proves to be extremely varied, for associated 

 with the felspar and biotite that form the bulk of the slide are 

 found rhombic and monoclinic pyroxene, hornblende, and quartz ; 

 apatite, magnetite, and zircon likewise occur. The plagioclase 

 is now more acid, with a mean refractive index of l - 560 : that is, 

 a labradorite of composition approximating to Ab,An r As 

 compared with the normal biotite, the hypersthene is much 

 less common — equalling in amount rather the pale-greenish mono- 

 clinic pyroxene. The last is often accompanied by a pale-green 

 hornblende that probably is original. In so basic a rock the 

 presence of free quartz is noteworthy. 



Towards the cen tre of the intrusion ,garnetiferous monzonite 

 appears— much coarser in texture, and with garnets the average 

 diameter of which reaches 1*3 cm. Quartz, felspar, and biotite 

 are other important constituents ; Avhile zircon, apatite, sphene, 

 indicolite, magnetite (often titaniferous), pyrite, and monazite 

 also occur. 



Among the felspars are found orthoclase and the two plagio- 

 clases, andesine and oligoclase. The plagioclases are occasionally 

 zoned, and in all there is considerable turbidity due to decomposition. 

 Chlorite is an alteration-product of the biotite, and often carries 

 sagenite ; it also accompanies the garnet. In this type occur 

 micropegmatitic intergrowths of quartz and orthoclase, both as 

 grains and as fringes to the felspar. At the same time the 

 development of mortel-struktur suggests the operation of 

 dynamic forces. 



The monazite occurs in small, biaxial, greenish-yellow, pleochroic 

 grains, having a maximum diameter of 0*18 mm. It was, how- 

 ever, possible to use a spectroscopic eyepiece, and identify the 

 didymium lines. 



