﻿Vol. 64.] ST. DAVId's-HEAD ' EOCK-SEEIES.' 289 



moulded (see PI. XXIX, fig. 2). It is of a rich red-brown, with 

 normal pleochroism, and is sometimes bleached by iron-separation. 

 The occurrence of this mineral seems to have been strictly 

 determined by the composition of the magma. JSTo inclusions have 

 been noticed in it. 



Hornblende. — Hornblende, like biotite, is but sparingly 

 represented, and, with rare exceptions, occurs only in the more 

 acid varieties. At Carnedd Lleithr it becomes abundant, and the 

 rock passes into the class of enstatite-diorites. It separated, in 

 most cases, later than the pyroxenes, and occurs both in separate 

 crystals and as an outgrowth upon augite. It is brown in colour, 

 and seems to be always a primary constituent. Secondary horn- 

 blende (as an alteration-product of augite) is very rare in these rocks, 

 and uralite is practically absent in all the slices. Some transparent 

 fibrous alteration-products after enstatite may possibly be rhombic 

 maguesia-amphibole, allied to the kupfferite described by Messrs. 

 Allen, Wright, & Clement,^ as the extinction is parallel to the 

 -elongation of the fibres. This point, however, requires further 



Apatite. — The distribution of apatite in these rocks presents 

 some features of interest. This mineral is not very conspicuous in 

 the basic varieties, but becomes rather a prominent constituent of 

 the acid types, in which it often forms crystals of considerable size. 

 As chlorine was found in the analysis of this type, the mineral 

 may possibly be in the form of chlor-apatite. It is noteworthy 

 that Mr. Harker observed a similar distribution of chlor-apatite 

 in the Skye gabbros, where it was only found in the more acid 

 varieties.^ 



In the St. David's-Head rocks this peculiarity of distribution is 

 also reflected in the separate slices, where the microscope shows it 

 to be largely concentrated in the quartz-areas, and to a smaller 

 degree in the felspars ; it is rarely found in the pyroxenes. I put 

 this curious fact to a rough quantitative test. Thus in sixteen rock- 

 slices it was found that the number of apatite-crystals recognizable 

 amounted to 1142, of which 824 were enclosed in quartz, 259 in 

 felspar, 26 in pyroxenes, and 33 in other minerals. Considering 

 the fact that quartz rarely forms more than 5 per cent, of the rock, 

 it appears remarkable that so large a proportion of the apatite 

 should be concentrated in the mineral which was the last to 

 consolidate. Apatite-inclusions can be so readily recognized in the 

 pyroxenes that they could scarcely escape detection. It is difficult 

 to account for this curious fact, but two explanations seem possible. 

 Either the separation of apatite was delayed by the presence of 

 some substance which increased its solubility ; or it was formed, as 

 usual, in the early stages of consolidation, but was rejected by the 



^ Amer. Journ. Sci. ser. 4, vol. xxii (190()) p. 406. 



2 ' Tertiary Igneous Kocks of Skye ' Mem. Geol. Surv. U. K. 1904 p. 113 



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