﻿454 PROF. E. W. SK.EATS ON THE GNEISSES [Aug. I9IO, 



characters up to the junction. It is clear that the gneiss cannot 

 be regarded as a marginal modification of the granodiorite, but is a 

 rock having a distinct mode of origin. Close examination of the 

 junction provided definite evidence, not only that the gneiss is 

 older than the granodiorite, but also that it received its foliation 

 before the final consolidation of the granodiorite took place. This is 

 demonstrated by the occurrence of acid veins allied to aplite and 

 pegmatite which traverse the gneiss at and near the junction with 

 the granodiorite, and often cut right across the foliation of the 

 gneiss. These acid veins show no signs of having undergone 

 disturbance since their intrusion into the gneiss. Several of them, 

 ranging in size from a quarter of an inch up to several feet in 

 diameter, occur at and near the junction of the granodiorite on a 

 ridge about a third of a mile south of Selby Station. In addition, a 

 large acid intrusion, about 100 yards long and 20 feet in minimum 

 breadth, is intercalated among the gneissic rocks of the ridge and 

 parallel to their planes of foliation. There can be no doubt, 

 therefore, that the rock which is now a gneiss became foliated 

 before the intrusion of the acid veins. 



In those parts of the contact where the gneisses are absent and 

 the dacite is but partly schistose, evidence is also obtainable that the 

 granodiorite is a later rock intrusive into the dacite series. South 

 of Upway and in one or two other localities acid veins at the contact 

 can be seen penetrating the slightly altered dacite. It is also to be 

 noted that a tourinaline-bearing pegmatite vein, a few inches thick, 

 traverses the fairly normal dacite in a railway-cutting immediately 

 east of the bridge near Belgrave Station. 



The relations of the granodiorite to the altered dacites and to the 

 gneisses are thus seen to be similar. In each case the plutonic is 

 the younger rock intrusive into the older series. 



Apart from the acid veins mentioned above, a few dykes of horn- 

 blende-porphyrite have been seen near Aura, both in the granodiorite 

 and in the gneissic and dacite series. These, of course, afford no 

 certain evidence as to the relative ages of the rocks which they 

 traverse ; but it is believed that they are genetically connected with 

 the granodiorite and form one of the later incidents connected 

 with the intrusion and consolidation of the plutonic rock. 



(e) Relations of the Gneisses to the Dacites. 



While the junction of the gneisses with the plutonic rocks is, 

 wherever seen, sharp and definite, no such clear demarcation has 

 been made out between the gneisses and the dacites. Rock-exposures 

 are not numerous, as the country is densely timbered and there is 

 generally a somewhat luxuriant undergrowth. In places, however, 

 clearings have been made ; while sometimes a forest fire serves the 

 same purpose, and then fairly continuous rock-exposures can be 

 seen. 



