﻿Vol. 64.] ST. DAVId's-HEAD ' ROCK-SEEIES.' 277 



rock show no evidence of orientation, from which it is concluded 

 that these bands consolidated after the motion, by which they were 

 produced, had ceased. This was not the case in other parts of the 

 mass, as, for example, along the coast between Carn Porth Llong 

 and Trwyn Llwyd. Here we find evidence that some amount of 

 movement was in progress during crystallization, in some parts a 

 marked flow-structure being seen (PL XXX, fig. 4), and in other 

 cases the crystals themselves having been broken and disturbed. 

 These evidences of movement have onh" been noticed in parts of 

 the St. David's-Head mass. In the Carn-Llidi intrusion I found no 

 appreciable signs of banding, flow-structure, or crystallization under 

 dynamic stress. 



The occurrence of banded structure has an important bearing 

 upon the origin of the different rock-types. Banded gabbros have 

 been described from many localities. They have been noted by Sir 

 Archibald Geikie and by Mr. Harker in the Cuillin Hills (Skye),^ 

 by the latter in the Carrock-Fell gabbro," and by numerous other 

 observers in various parts of the world. The structure seems to be 

 best explained by fluxion in a heterogeneous magma; and the con- 

 clusion is, therefore, drawn that, in that, part of the St. David's- 

 Head intrusion where this structure is exhibited, an acid ^ and a 

 basic magma coexisted in parallel bands, which consolidated before 

 diffusion had time to reduce them to a state of homogeneity. 



In the Carn-Llidi mass, although there seems to be no definite 

 banding, there are acid and basic streaks, forming irregular 

 lenticles, roughly aligned in the general direction of the intrusion. 

 In many parts more or less perfect mixture seems to have taken 

 place, the extreme types being then scarcel)' recognizable. 



It is possible that local variations in viscosity pla^^ed the chief 

 part in causing these differences. Viscosity is determined not only 

 by temperature, but also by the composition of the magma, the 

 presence of magmatic water, and other causes, all of which factors 

 might vary locally. Extreme fluidity, aided by a high temperature 

 or other causes, would tend to promote diffusion and homogeneity. 

 This result seems to have been largely realized in the neighbourhood 

 of Carn Twlc and Carnedd Gwian, while at Pen Lied wen, and in 

 parts of Carn Llidi and St. David's Head, schlieren are more in 

 evidence, as if the magma here was in a more pasty condition. 



III. Characters or the Rock-Types. 



In order to arrive at a more intimate knowledge of the different 

 rock-types exhibited in this area, not only were thin slices prepared 

 for microscopic examination, but a representative specimen of each 



1 ' The Tertiary Igneous Eocks of Skye ' Mem. Geol. Surv. U. K. 1904, 

 p. 91. 



2 Quart. Journ. Geol. Soc. vol. 1 (1894) p. 319. 



^ The term ' acid ' is here used to denote rocks containing primary quartz. 

 The silica-percentage is rarely high enough to entitle these rocks to a place in 

 the acid group as generally understood. 



