116 AUSTKALASIAN ANTARCTIC EXPEDITION. 



one must question the term " normal gabbro " for the least altered members of the 

 series. In the description of the normal gabbro it is stated that the felspar contains 

 cracks due to incipient fracture and crushing, as well as cloudy spots of saussurite ; that 

 the augite appears as diallage and brown hornblende is often associated with diallage ; 

 that the olivine " weathers " to serpentine and magnetite, and sometimes to dense 

 aggregates of talc ; that hypersthene is rare and only seen as thin borders to clusters 

 of olivine ; that there are reaction rims of diallage or hypersthene around the olivine 

 and of a fibrous radiate mineral between felspar and olivine ; that there is a typical 

 gabbroid structure. It is well to remember that Grubenmann has stated * that the 

 gabbroid structure may be a variety of the granoblastic structure. The presence of these 

 characters inclines us to view the so-called " normal gabbro " not as an absolutely 

 unmodified igneous rock. Now Grubenmann recognises | the difficulty in the separation 

 of certain crystalline schists with massive texture from igneous rocks. But, however, 

 if the gabbros are pre-serpentinisation in age, and if the schistositv be produced in 

 the manner suggested, we would scarcely expect normal gabbro to remain as such, 

 and we might expect part of the gabbro to be involved in pressures of the hydrostatic 

 type. It may, therefore, be better to accept the small amount of evidence and 

 recognise the affinity of the " normal gabbros," as well as the flaser gabbro and the 

 gabbro schists, to the metamorphic types. If, on the other hand, the igneous character 

 be maintained as dominant, the small area at Coverack must not be looked upon as 

 normal, but as a relic of the original gabbro. The metamorphic, not the igneous, state 

 is here the truly normal character. The troctolite at Coverack possesses saussuritised 

 felspar, serpentinised olivine, and reaction rims, and, therefore, also possesses meta- 

 morphic traits. 



There remain for comment the Kennack gneisses (pp. 119, et seq.) which Flett and 

 other workers consider to be the crux of the Lizard problem, though we think that the 

 serpentine has a claim to that distinction. The Kennack gneisses, however, present 

 a distinct problem in themselves. Flett has brought forward strong evidence to show 

 that these gneisses are metamorphosed igneous intrusions in the form of stocks, sills, 

 dykes, veins, and networks into the original peridotite. If they be pre-serpentinisation 

 they would necessarily be subjected to the same metamorphosing action as the gabbro 

 dykes. The dykes will be foliated parallel to their length and the stocks parallel to their 

 margins. Any peridotite blocks that had been caught up by the invading magma 

 before the serpentinisation would yield to the serpentinising forces in the same manner 

 as the large mass. Presence of the serpentine inclusions in the sills or gneiss is not 

 necessarily evidence that serpentinisation occurred before the intrusions of the gneisses. 



The complexity of the Kennack gneisses is due to the mixture of primary basic 

 rock and primary granite rock. With regard to the " Blocky Gneisses," in which blocks 

 of basic rock are included in granite gneiss, the fact that the blocks are seldom angular, 

 but are usually rounded or lenticular, does not show that they were being dissolved by 

 the granite magma. Where Flett records the evidence of acid magma diffusing into the 



* Op. oit.. voL L, p. 79. t Op. oit., vol. II., p. 19. 



