370 H. H. Read — The Two Magmas of Strathbogie. 



gradual passage from one rock to another can be demonstrated and 

 the individual rock types are well marked. Again, the west-east 

 sequence in certain localities may be partly reversed. Moreover, 

 there is no ultrabasic western border to the norite north of the 

 Huntly district. It seems more probable that the differentiation of 

 a gabbroic magma took place in a lower chamber and that the 

 picrite was intruded as a small sill, followed by increasingly acid 

 derivatives, each of which slid in roughly on the top of its fore- 

 runner. But most of the picrite-norite set were fluid at the time of 

 intrusion of the whole series and so no marked contacts are seen - f 

 schlieren and banded types occur and mixed edges ma)^ have been 

 formed locally. Finally, the phase of the picrite-norite set was 

 ended by the widened sphere of sill-like intrusion, together with 

 great cross-cutting, of the norite magma, and by the time of intrusion 

 of this norite the more basic members were sufficiently solid to be 

 cut by dyke-like apophyses of gabbro and norite. The areal 

 relations of this set are: picrite "83 sq. mile, olivine-gabbro- 

 •39 sq. mile, troctolite 3 - 32 sq. miles, and norite 37'4 sq. miles. 



After the composite intrusion of the picrite-norite set had finished,, 

 there were intruded a few small bosses of diorite (quartz, biotite, 

 augite, andesine). Of these bosses, that at Gibstone (1+ miles N.W.. 

 of Huntly) has a marked foliation which is, however, not of 

 cataclastic origin. This Gibstone diorite is intermediate in composi- 

 tion between the earlier norite and the later monzonite, and probably 

 owes its foliation to its soft and hot nature at the time of the move- 

 ments which caused the monzonite to present a markedly intrusive- 

 character to the norite. 



The picrite-norite set was solid at the time of intrusion of the 

 large monzonite mass of the Bin Wood (Central Intrusion of Watt 1 ). 

 This rock is a coarse garnetiferous monzonite which has produced 

 great contact effects on the earlier norite. 2 It consists of orthoclase, 

 plagioclase, augite, biotite, quartz, and garnet. 



The next member of the Younger Series is of granitic nature and 

 forms small masses, sometimes in the basic members and sometimes 

 as isolated bosses, as shown on the Sketch-map. The rock is a grey 

 granite composed of quartz, predominant microcline, orthoclase, 

 scarce plagioclase, biotite, and scarce muscovite. It often shows 

 a very rude fluxional structure. All the granites associated with 

 the Huntly Mass are biotite-microcline granites. 



Tourmaline pegmatites are common as the final stage in the 

 intrusion of the Younger Series of the Huntly Mass. 



The areal distribution of the whole Younger Series of this area is: 

 ultrabasic *83 sq. mile, basic 41 sq. miles, intermediate 2 - 8 sq. miles, 

 and acid rocks 32 sq. miles. 



Continuous variation in place is not seen in the Huntly Mass, and 

 there seems to be an individuality about the various closely related 

 types which is not explained by a crj'stallization-differentiation 

 hypothesis. 



1 W. E. Watt, loc. cit., p. 275. 



2 W. E. Watt, loc. cit., pp. 282 ff. 



