11-i ME. C. E. TILLEY 0?^ THE GEANITE-GIs^EISSES [vol. IxXVU, 



in Avliicli no unidirectional foliation can be observed are often 

 quite irregular in shape, and are penetrated at liapliazard by the 

 invading magma. In some cases this massive texture has been 

 such as to resist almost completely the efforts of the magma to 

 invade it. Those inclusions which have yielded by plastic deforma- 

 tion to the flowing movements in the gneiss have sometimes 

 quite remarkable and intricate shapes. 



Whether an amphibolite inclusion shall show signs of plas- 

 ticity is doubtless dependent on more than one factor : for example, 

 the length of time during which the inclusion is suspended in 

 the liquid magma (and so the extent of thermal metamorphism), 

 the severity and magnitude of the movements in the flowing 

 gneiss, and the degree of metamorphism impressed on the rock 

 prior to its disruption from the parent mass, may be considered 

 valid indicators of the extent of plastic deformation. Examples of 

 plastic deformation following on 1 it-par-lit intrusion can be seen 

 in many bands. Here pegmatite strings have been contorted and 

 drawn out into isolated lenticles, though plasticity may be well 

 developed in masses which show but little sign of magmatic 

 intrusion. 



In the foregoing paragraphs the interpretation of the amphi- 

 bolites, as a whole, as metamorphosed igneous rocks is of some 

 assistance in any attempt to locate the source of these inclusions. 



If we look to the series of pre-existing sediments now highly 

 metamorphosed — the Hutchison Series — for the source of these 

 amphibolites, there is found no adequate supply. There are, it is 

 true, among the sediments some basic dykes and sills ; but these 

 form quite an inadequate percentage of the series. Moi'cover, it 

 is not known whether these dyke-rocks are not ' post-gneiss ' in 

 origin. In the absence of any definite area now exposed to view 

 for the source of these inclusions, an attempt to trace their history 

 further back must be somewhat speculative, but from general 

 considerations I ma}'" offer suggestions which appear to have some 

 plausibility. 



It may be suggested that the amphibolite inclusions represent 

 the disrupted portions of an older igneous terrane, which was 

 intruded in a deep-lying portion of the crust, into the pre-existing 

 countrj'- rocks, possibly the members of the Hutchison Series of 

 which only fragmentary remains are now visible, or some older 

 series still of which there is no record. In general these amphi- 

 bolites are of gabbroid or doleritic composition, so that in the 

 older intrusions, from which these inclusions are derived, gabbroid 

 rocks were largely represented. It seems the more probable that 

 these earlier gabbroid intrusions belonged to the same orogenic 

 epoch, or diastrophic period, as that which finally culminated in the 

 irruption of the granite-gneisses. On this view the igneous 

 equivalents of the amphibolites are thus comagmatic with the acid 

 granites. 



It is not difficult to conceive of earlier and deep-lying intrusions 



