268 mr. Pv. h. rasiall on the [May 1906, 



which is partly still fresh colourless augite, but for the most part 

 converted into hornblende by uralitization. Some of the hornblende 

 is actinolitic, in aggregates of long slender needles, but much of 

 it is so well crystallized as to resemble an original constituent ; it is 

 distinctly pleochroic (bluish-green to pale yellowish-brown), and 

 some patches have a very distinct blue colour. There are also a few 

 flakes of a bright reddish-brown biotite, and abundant patches of 

 ilmenite exhibiting very conspicuous bar-structure. 



In the centre of this dolerite-intrusion is a large patch of a very 

 coarse-grained gabbroid modification [4774], which shows crystals 

 of felspar and uralitized augite measuring up to half an inch in 

 length, and plates of ilmenite measuring up to 1 inch in diameter. 

 Scattered through it are abundant flakes of red-brown biotite, 

 measuring up to a quarter of an inch in diameter. Another note- 

 worthy feature of this rock is the occurrence of what, at first sight, 

 look like orbicular patches. A closer examination shows that these 

 are partly-digested xenoliths, since every gradation can be traced — 

 from obvious angular inclusions, up to more or less regular ' oroides' 

 measuring an inch or more in diameter. 



No quartz was observed in any part of this dolerite-mass, and it 

 is probably the most basic rock yet examined in this series. 



IV. General Characters. 



It appears from the foregoing descriptions, that this great series 

 of intrusions comprises a large number of rock-types of variable 

 character, but still showing many points of resemblance among them- 

 selves. Speaking generally, they may be regarded as products of the 

 differentiation of one original magma ; and this differentiation seems 

 to have taken place in two stages, the deep-magmatic and laccolitic 

 phases of Prof. Brogger. There has also occurred, to a certain extent, 

 production of peculiar rock-types, as the result of the subsequent 

 admixture of previously-differentiated partial magmas of the deep- 

 magmatic series. 



In both stages, the amount of basic rock produced by differentiation 

 is small, in comparison with the bulk of acid rock ; and this suggests 

 that differentiation only continued for a short time, and the variations 

 produced are not extreme. 



If we consider the mineralogical composition of the magma as a 

 whole, without regard to small basic modifications, it will be seen 

 to consist of quartz, an intermediate plagioclase near to oligoclase, 

 with subordinate orthoclase and perthite, and a rather small pro- 

 portion of a ferromagnesian constituent (which crystallized either as 

 biotite or augite, according to circumstances). Such a magma shows 

 more resemblance to the tonalites than to the granites, although it 

 is rather more acid than the typical tonalites, which usually contain 

 about 66 per cent, of silica. 1 According to Clifton Ward 2 this rock 

 contains about 71*5 per cent. 



1 H. Eosenbusch, ' Elemente der Gresteinslehre ' 2nd ed. (1901) p. 144. 



2 Quart. Journ. Geol. Soc. vol. xxxii (1876) p. 22. 



