A. Somervail — Schists of the Lizard District. 165 



opinion is entirely based on field observations, and on the macro- 

 scopic aspects of the rocks, and also on the absence of any positive 

 proof as yet furnished by the microscopists to lead to an opposite 

 conclusion. 



V. Eelations between the two Groups. 



There is also considerable diversity of opinion prevailing with 

 regard to the relations between both groups, and the rocks which 

 compose each, especially the rocks making up the "granulitic." 



General McMahon in his paper referred to speaks of the " granu- 

 litic " group as containing many intrusive rocks, injected at different 

 times, such as ordinary diorites, also a later porphyritic diorite, and 

 still later veins of granite which traverse the whole. 



It seems to me that if we were to eliminate both from the "horn- 

 blende " and " granulitic " groups these injected diorites and granite 

 veins, we would almost, if not altogether, remove the whole of the 

 rocks composing them. The porphyritic structure even in the horn-> 

 blende group is much more common than Was suspected, and I think 

 it is by no means certain that there is any distinction in points of 

 origin and age between the porphyritic and non-porphyritic varieties, 

 even in both of the groups. Neither do I think it certain that the 

 so-called granite vein portion of the " granulitic " group, such as 

 we see at Kennack and elsewhere, has had a distinct and sepai'ate 

 origin from the diorites. 



Mention of the fact has already been made that the rocks making 

 up the " hornblende " and " granulitic " groups vary from a diorite 

 to one of granitic type, but between these two extremes there are 

 however other rocks of an intermediate nature forming what I 

 regard as a transition between both of these extremes, and associated 

 with them in such a way as to bind them together into one natural 

 group, having a common age and origin. 



There are certainly areas, such as the foreshore of Kennack Cove, 

 which might be selected as proof of the intrusion of the granite- 

 like rock into the diorite; but, even here, it can equally well be 

 read in the reverse way, so confusedly mingled together are these 

 two varieties of rocks, and so often do they alternate with each 

 other. As soon however as we leave the foreshore for the cliff of 

 serpentine, we find both of these rocks combined together in varying 

 proportions thrust through it. Not only is this so in several of the 

 dykes in the Kennack area, but in many dykes throughout the 

 whole of the Lizard district. In the dykes these rocks are so 

 mixed up and banded together that some other explanation than 

 their separate intrusions must be sought for. In my opinion this 

 explanation is found in the separation of the basic and acidic 

 portions of the magma while in the act of cooling, as pointed out in 

 my short paper on the North Pentreath dyke ; l or it may be the 

 case that, during the eruption of these rocks, acidic and basic 

 magmas were being simultaneously belched forth together, pre- 

 serving a partial separation ; but, as we shall presently see, there 



1 Geol. Mag. Decade III. Vol. V. p. 553 (1888). 



