WELEELE ee montana ted Gabbro in berdecnshine IGT 
3. Several tension cracks may have opened at one time. During 
a long experience in testing various kinds of materials by slowly 
loading them in tension, I only remember two cases where the test 
plece gave way at more than one surface. Both cases were very 
brittle cast-iron. On the other hand, if a piece of material in tension 
be subjected also to a sudden blow, it is very lable to go at 
more than one plane. This is seen in the sudden tearing of cloth, 
where in addition to the main tear, several of lenticular shape and 
parallel to the main tear frequently appear. Ifthe intrusion occurred 
at a time of anticlinal tension and the molten material below 
subjected the crust above to a sudden impulse, the rupture, instead 
of being confined to one plane, would oscillate over a small region, 
finally causing fracture at a number of closely spaced vertical planes. 
The main mass of magma rushed up one of these fissures, and at 
the same time a smaller amount filled the adjoining cracks. The 
uniform width of the sheets and the length over which they are 
exposed suggests suddenness of action, for if the process had been 
prolonged the main dyke might have been expected to transgress 
from one fissure to another in different parts of its course. 
This explanation is of interest in the light of J. Barrell’s theory, 
based on thermodynamic grounds, that dyke injection has often 
proceeded in a violent and explosive manner. 
The Contaminated Gabbro of Easter Saphook near 
Old Meldrum in Aberdeenshire. 
By H. H. Reap, H.M. Geological Survey, Scotland. 
INTRODUCTION. 
N North-East Scotland the igneous rocks have been divided 
into two series, whose times of intrusion were separated by the 
movements responsible for the foliation and disposition of the 
crystalline schists of that area.2 With the older series, intruded 
prior to or during those movements, this paper is not concerned. 
The younger, or non-foliated, series supplies rock types ranging from 
peridotite to granite. Gabbro is the chief rock in the large 
independent basic masses formed by the younger igneous rocks ; 
to the more important of these masses may be given the names of 
the Huntly, Insch, Boganloch, Haddo, Arnage, Maud, and Belhelvie 
Masses. For the most part they lie within Sheets 76, 77, 86, and 87 
of the 1 inch Geological Survey Map of Scotland. 
This paper is concerned with a group of rocks which can be shown 
to result from the incorporation of sedimentary material in the 
gabbro magma. Chief amongst the rocks resulting from the mixture 
1 J. Barrell, U.S.A. Geol. Surv., Prof. Paper, No. 57, p. 157, 1907. 
2 H. H. Read, Swmmary of Progress for 1918, 1919, p. 24. GOL. MAG., 
Vol. LVII, 1919, p. 364. Swmmary of Progress for 1919, 1920, p. 28. 
VOL. LVIII.—NO. IV. 12 
