﻿Vol. 64.] ST. DAVId's-HEAD ' EOCK-SERIES.' 281 



scarcely reach the composition of andesine. Some zoning of the 

 felspars may generally be noticed in the neighbourhood of quartz- 

 areas, the inner zones being apparently more basic than the later 

 separations. There is no biotite, but some hornblende occurs as an 

 accessory constituent. The proportion of apatite is generally some- 

 what greater than is indicated in the particular sample selected for 

 analysis. 



This rock appears to be a typical enstatite-bearing quartz-gabbro. 

 It is often very coarse in grain. The proportion of rhombic to 

 monoclinic pyroxene varies greatly in different specimens, as also 

 does the quantity of free silica, v^^hich is rather less than the 

 average in the specimen analysed. It is probable that the greater 

 viscosity of this type facilitated clotting. There is evidence, also, 

 as mentioned above (p. 277), that some movement v^as going on in 

 parts of the magma during solidification. These causes alone would 

 account for considerable local differences in composition. 



Illustrations of this type are shown in PI. XXIX, fig. 5 & 

 PL XXX, fig. 3. 



(c) The Intermediate Type. 



Excluding the aplite-veins, the above-described rocks seem to 

 form respectively the basic and acid extremes of the main mass of 

 these intrusions. A considerable proportion of the rock, however, 

 has intermediate characters, and differs from the basic type, on the 

 one hand, by the presence of a little more silica and the absence of 

 biotite ; and from the acid rocks, on the other, by the scarcity of 

 hornblende and quartz, although a little of the latter mineral 

 (probably in some cases of secondary origin) is sometimes visible. 

 Apatite, although more conspicuous than in the basic type, is less 

 abundant than in the acid varieties. There seems to be, also, a 

 gradual passage from this intermediate variety into each of the 

 extreme types, making it somewhat difficult to select a typical 

 example for analysis. The specimen taken for this purpose is a 

 rather compact, fine-grained rock from the eastern flank of Carn 

 Llidi, about 10 yards from the margin. 



The following is the result : — 







Molecular 





Per cent. 



projyortions. 



SiO, 



50-55 



842 



Tio; 



1-58 



19 



A1203 



15-00 



144 



FeoO, 



2-54 



15 



Feb:. 



7-90 



109 



CaO 



7-85 



140 



MgO 



6-25 



156 



Na^O 



3-53 



56 



K,6 



1-10 



11 



p'o, 



n. d. 





H.,d- 



0-55 





H,0-f 



3-14 



Specific gravity = 







Total 



99-99 





2-92 



