﻿Vol. 64.] THE ST. DIVId's-HEAD ' EOCK-SERIES.' 275 



and occasionally at Pen Lledwen and in parts of the St. David's- 

 Head mass. This melanocratic type is moderately coarse in grain. 

 It stands in conspicuous contrast to a leucocratic variety, often 

 much coarser in grain, with abundant felspar and more or less 

 quartz. This latter type forms only occasional streaks in the Carn- 

 Llidi mass, but in the St. David's-Head mass it occupies a far 

 larger area and becomes the predominant variety. It is especially 

 prominent in the strip adjoining the coast in a north-easterly 

 direction from St. David's Head. 



In addition to these apparently extreme varieties, a considerable 

 bulk of the masses seems to be of an intermediate character, some- 

 what variable in colour and texture, but easily distinguishable in 

 the field from either of the above-mentioned types. This forms the 

 predominant rock in the Carn-Llidi mass, but in the St. David's- 

 Head mass it is subordinate to the more acid type. Einally, there 

 are some thin veins of a fine-grained rock looking like an aplite. 

 These, however, do not seem to be very abundant. 



The above-described types of rock might easily be looked upon as 

 a normal result of diff'erentiation in situ, as typically exemplified in 

 the gabbro of Carrock. Fell, and in many other cases where basic 

 rooks exhibit a coarse-grained acidic core. A more detailed study in 

 the field, however, scarcely supports this view. There is not only 

 no border-segregation in the mass as a whole, but the acid type 

 seems to have a somewhat capricious distribution, passing oc- 

 casionally by very sudden transitions into the more basic varieties, 

 among which it can sometimes be seen to lie in irregular bands, 

 having a fairly-sharp line of demarcation. These acid streaks are 

 not confined to the heart of the mass, but occasionally extend into 

 the neighbourhood of the marginal contact- zones. In a general 

 sense, however, they conform to the strike of the intrusions. The 

 proportions in which the extreme types occur are locally very 

 difi'erent, the more acid rock being far more abundant than the 

 basic rock in the westernmost part of the Carn-Llidi mass, and in 

 the eastern part of the St. David's-Head mass. The acid rock, in 

 short, does not appear to represent merely the residual mother- 

 liquor occupying the core of the intrusion. 



It is desirable at this stage to examine the field-relations of 

 these rocks in greater detail. The shales at the junctions of the 

 intrusions are indurated, but no very extensive mineral alteration 

 is apparent in hand-specimens. The igneous rock at the contact 

 has a fine-grained sahlband, which can be followed for a distance 

 of several feet into the mass. Microscopic examination shows that 

 some assimilation of the country-rock has taken place. The 

 character of the intrusion is considerably modified at the margins. 

 The felspars, in small ragged laths, have an approximately-straight 

 extinction. They are apparently more acid than those of the main 

 mass of the rock, and appear to be oligoclase. Augite in small 

 crystalline grains is abundant, and the iron-ores, which are but 

 moderately developed, usually have the appearance of magnetite. 



