H. H. Read—Contaminated Gabbro in Aberdeenshire. 179 
CONTAMINATED GABBROS OF HASTER SAPHOOK. 
Easter Saphook is situated at the extreme end of the Insch Mass, 
some 24 miles west by north from Old Meldrum. The contaminated 
gabbros outcrop in several small knolls around the copse a few 
hundred yards south of the farm. Small quarries opened in these 
knolls provide excellent specimens of the mixed rocks. Just east 
of this locality various types of sedimentary gneisses are seen near 
the burn which runs from north to south a few hundred yards east 
of Haster Saphook. The mixed rocks, therefore, occur at the margin 
of the igneous mass. 
The uncontaminated igneous rock of this part of the Insch Mass 
is a fairly uniform hypersthene-gabbro.! It is a fine-grained greyish 
rock, weathering into rectangular slabs. In thin slice the texture 
is seen to be uniformly equigranular. The constituent minerals are 
labradorite, hypersthene, augite, and magnetite. The felspar occurs 
in stout crystals. The two pyroxenes are about equal in amount 
and are oftenintergrown. The hypersthene builds vividly pleochroic 
prismatic grains; the augite forms similarly shaped grains with a 
well-marked diallagic parting. 
Around Easter Saphook, however, the uniformity which else- 
where characterizes the hypersthene-gabbro is lost. Here the rocks 
become very variable, all degrees of textural and mineralogical 
variations being seen in the same exposure and often in the same 
hand-specimen. Hornfelsed sedimentary blocks can be hammered 
out of the contaminated igneous matrix. It is at once apparent that 
here are assimilation phenomena similar to those repeatedly 
observed in the Huntly Mass. 
At Easter Saphook the earliest stage in contamination is repre- 
sented by a hypersthene-gabbro, which, whilst in the main similar 
to the normal rock of that group, yet shows peculiar features, 
indicative ofits mixed origin. These least modified types are medium- 
grained greenish rocks, with little of that variability shown by more 
modified examples of this suite. Their constituent minerals are 
labradorite, hypersthene, scarcer augite, biotite, quartz, magnetite, 
and secondary green hornblende. Evidences of modification are 
provided by :— 
(i) The presence and skeletal arrangement of the biotite, which is 
always near iron-oxide grains. 
(ii) The occurrence of scattered areas of quartz, which often 
forms patches, blebs, and points in the felspar, and then the two 
minerals tend to occur in regular granophyric or micropegmatitic 
intergrowths. 
(1) The purplish grey appearance of the hypersthene.? 
(iv) The tendency of the hypersthene to form a collection of 
grains rather than homogeneous crystal plates. There is a rude 
orientation of felspar and hypersthene, pointing to movement 
during consolidation. 
a lbidaspewAas 
2 W. RB. Watt, loc. cit., pp. 282, 286. 
