488 J. J. Harris Teatt—The Lizard Gabbros. 



origin of this foliation. Is it an original or secondary structure? 

 That it is due to a differential movement in the mass after the separa- 

 tion of the individual constituents will I think be admitted on all 

 hands. The only question that can arise is whether this movement 

 was in any way connected with the intrusion of the rock. The facts 

 appear to me to point very decidedly to the conclusion that it is a 

 secondary structure due to earth-movements acting upon the solid 

 rock. That the Lizard district has been profoundly affected by earth- 

 movements is apparent on every hand. The rocks have been folded, 

 faulted, crumpled, twisted, and in some places brecciated by the 

 intense pressures that have acted upon them. The evidences of 

 mechanical disturbance are not limited to any one variety of rock or 

 to any one district. They abound in every portion of the district, 

 though their effects are more pronounced in certain localities than in 

 others. If we attempt to determine the precise nature of the earth- 

 movement, we are met with considerable difficulty on account of the 

 superposition of the effects of distinct movements. The earlier 

 movement, and the one which I suspect produced the greater part of 

 the foliation in the gabbros, appears to have acted so as to produce 

 a strike about N.W. and S.E. or N.N.W. and S.S.E. This earlier 

 movement was, however, certainly followed by one producing faults 

 which run approximately in an E. and W. direction. The latter 

 direction agrees with one of the great post-Carboniferous disturb- 

 ances, and the faults which run in this direction probably belong to 

 those disturbances. That the two sets of earth-movements above 

 referred to are the only ones that have acted upon the district I am 

 by no means prepared to say ; but that these two have acted is shown 

 by facts which may be observed at Porthalla, George Cove and 

 Kynance. 



That the foliation in the gabbros is one of the results of the 

 pressure- or regional-metamorphism which has operated upon the 

 district, is rendered at once probable when we compare the Lizard 

 gabbros with those of the West of Scotland. The latter consist 

 mainly of felspar-diallage rocks, and are not foliated ; the former are 

 largely composed of saussuritic and hornblendic rocks, and are often 

 conspicuously, foliated. The Lizard district has been profoundly 

 affected by earth-movements, whereas the Tertiary volcanic district 

 of the West of Scotland has not been so affected. 



This argument is in itself not conclusive, but it becomes greatly 

 strengthened when we consider the distribution of gabbros of the 

 Lizard type. They have been described by Dr. Keusch l from the 

 Bergen Peninsula, by Prof. Lehmann 2 from the granulitic region of 

 Saxony, and by Mr. Hatch 3 from the Tyrol. Each one of these 

 districts gives independent evidence of having been subjected to 

 pressure-metamorphism. Perhaps the most convincing proof that 

 the structure is the result of movement after the consolidation of the 



1 Die Fossilien fiihrendcn krystalliniscken Schiefer von Bergen in Norwegen, 

 Leipzig, 1883. 



2 Die Entstehung der altkrystallinisehen Schiefergesteinen, Bonn, 1884, p. 190. 



3 Tsch. Min. Mitth, Band VII. (1885), p. 75. 



