Vol. 52.] 



ROCKS OP THE LIZARD DISTRICT. 



43 



disseminated iron oxide. Two or three dykes of coarse gabbro, 

 very poor in the ferro-magnesian silicates, have indeed been 

 observed on the coast farther south ; can this epidosite have been 

 a very compact variety of the same rock ? 



Manacle Point and Porihoustock Cove. — We can add a few notes 

 to the description given by Gen. M c Mahon and myself. 1 The 

 ordinary gabbro (which forms the Crousa Down massif) when 

 followed along the shore towards Manacle Point, passes occasionally 

 into a very coarse variety, in which the constituent crystals some- 

 times even exceed 2 inches in length. This forms patches in the 

 ordinary rock, with ill-defined boundaries, suggestive of an imperfect 

 mixture of heterogeneous materials in different stages of consolida- 

 tion. Other parts of the gabbro are foliated, and these occur in 

 like manner, so as to suggest local movements anterior to complete 

 consolidation. The ' warm grey ' or brownish rock intrusive in 

 the gabbro, 2 especially as Manacle Point is approached, would 

 repay a closer study than our arrangements allowed us to give it. 

 It pierces and rips up the ordinary gabbro in a very curious fashion, 

 and pieces of the latter sometimes assume singular shapes, as if they 

 had been slightly softened and bent (fig. 11). Frequently, also, it 



Fig. 11. — Strip of moderately coarse gabbro included in granular 

 dolerite (or gabbro), south of Porihoustock Point. 



The unmarked part indicates the latter rock. The sketch is diagrammatic. 

 The inclusion represented measures nearly 2 feet from end to end. 



includes flake-like fragments of the foliated variety. 3 Besides this, 

 it rather often exhibits a porphyritic structure, the mineral being 

 commonly felspar, but sometimes a variety of pyroxene, which 

 forms green spots. The felspar-crystals are a dead-white colour, 

 like that in the gabbro, ranging up to about half-an-inch in length. 

 They are ' sporadic ' in habit, occurring in small * swarms/ some- 

 times more or less scattered. Sometimes, also, they are actually in 

 contact, and it becomes difficult to distinguish them from the smaller 

 rock-fragments. Occasionally a felspar, as it appears at first, proves 

 on examination to contain a speck of diallage. In short, the 



1 Quart. Journ. Geol. Soc. vol. xlvii. (1891) p. 491. 



2 It is noticed on pp. 491 and 494 in the paper by Gen. M c Mahon and 

 myself. As there stated, it is a kind of dolerite or fine-grained gabbro. 



3 The bearing of these observations on some of the reasoning in the earlier 

 part of this paper will, I presume, be obvious. 



