180 A. H. Read—Contaminated Gabbro in Aberdeenshire. 
Further stages show increasing divergence from the igneous rock 
until the main characters of the mixture become dependent more 
upon the sedimentary than upon the igneous components. This 
divergence is exemplified firstly by a group containing much quartz, 
and secondly by a group containing cordierite. 
In the’ first branch the components are andesine, quartz, 
hypersthene, biotite, garnet, magnetite, and zircon. Chlorite and 
hornblende are of late secondary formation. The felspar forms 
euhedral crystals; quartz is abundant; hypersthene commonly 
occurs in small grains vividly pleochroic’in red, yellow, and biue- 
green tints. Yellow to red biotite builds large plates usually 
with the skeletal form ‘characteristic of this group of rocks ; 
garnet occurs associated with biotite and magnetite. 
A further stage along this line of divergence is possibly provided 
by a rock which has felspar, quartz, and biotite as 1ts.main con- 
stituents. The turbid zoned felspar varies from oligoclase to 
andesine; the biotite is yellow to black and forms ragged plates ; 
quartziscommon. Prisms of dark bluish-green tourmaline are note- 
worthy. This mixed rock is very like the Kennethmont quartz- 
diorite 1 in its quartz, biotite, and felspar, but lacks the green horn- 
blende of the true quartz-diorite. It is, of course, possible that the 
original igneous rocks of Saphook had as a member a rock already 
somewhat differentiated from the main gabbro type before con- 
tamination had occurred. Such a possibility, whilst by no means 
to be neglected, is rendered less likely by our experience of the very 
variable contaminated rocks of the Huntly Mass. 
The second line of divergence from the normal hypersthene- 
gabbro is characterized by a very variable group containing 
cordierite. At Saphook it is impossible yet to trace that detailed 
destruction of hypersthene and the formation of biotite, cordierite, 
garnet, etc., so well seen in the Huntly Mass. The Saphook 
cordierite-bearing rocks are nearer the sedimentary than the igneous 
end of the series. 
A member of this cordierite-bearing group is a rock made up in 
hand-specimen of black biotite plates and pinkish felspar in streaky, 
almost gneissose, association. In thin slice, the biotite builds large 
yellow to brown-black plates, usually ragged and often showing the 
skeletal or rudimentary forms characteristic of this suite. Cordierite 
is abundant in small grains, and shows magnificent pleochroic 
haloes and common sector-twinning. Amongst the felspars are found 
microcline, orthoclase, and andesine. Sillimanite forms fibrous 
needles in the cordierite and stouter blades in the biotite ; greenish 
spinel and clots of magnetite complete the slice. An allied type is 
composed of quartz in patches and grains, abundant cordierite in 
large, slightly pinitized grains, pink garnet, yellow-red ragged biotite, 
and grains of andesine and magnetite. These abnormal rocks show 
1H. H. Read, Summary of Progress for 1919, 1920, p. 25. 
