464 PROF. T. G. BONNEY AND MAJOR-GEN. C. A. M^MAHON 



26. Results of cm Examination of the Crystalline Hocks of the 

 Lizard District. Ey Professor T. G. Bonney, D.Sc, LL.D., 

 F.R.S., Y.P.G.S., and Major-General C. A. M*=Mahon, F.G.S. 

 (Read April 22, 1891.) 



[Plate XVI.] 



Contents. 



Page 



I. Introductory 464 



11. The Serpentine 465 



(1) Its Origin and Composition. 



(2) Its Kelation to the older Kooks. 



(3) Its Structures. 



III. The E-ocks older than the Serpentine 476 



(1) The Graniilitic Group. 



(2) The Hornblendic Group. 



(3) The Micaceous Group. 



IV. Igneous Rocks newer than the Serpentine 482 



(1) The Troctolite. 



(2) The Gabbro and its Structures. 



(3) Varieties of Gabbro. 



V. Manacle Point and Porthoustock Cove 491 



VI. Other Intrusive Rocks 492 



VII. Some Fragmentary Inclusions 495 



VIII. Summary of Results 497 



I. Introductory. 



Geologists during the last decade have devoted much more attention 

 to the rocks of the Lizard than in previous years. At the same 

 time the effects of pressure as an agent of metamorphism, especially 

 in producing foliation and simulating stratification, have become 

 increasingly appreciated. Hence it is not surprising that the 

 correctness of the view advocated by Prof. Eonney in 1883 *, viz., 

 that the crystalline schists of the Lizard were in the main of 

 sedimentary origin, has been contested, some writers even going so 

 far as to express doubts whether the relation of these masses to 

 the serpentine, gabbro, and other rocks of igneous origin had been 

 correctly understood. 



The results of work in other regions had raised suspicions in the 

 author's own mind as to the trustworthiness of some of the data on 

 which his inductions had been founded, and after spending several 

 days in 1886 at the Lizard with Mr. Teall, who was more disposed 

 than himself to regard pressure as a potent ager^tof metamorphism, 

 he felt convinced that his earlier views would have to be modified, 

 though to what extent could only be determined by further study 

 of the district and by comparison with other regions. 



In 1889 a paper was read before this Society by Gen. 

 M^'Mahon, in which the igneous origin of many of the foliated 

 crystalline rocks was maintained, and an hypothesis put forward to 



* Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 1. 



