﻿Vol. 64.1 ST. DAVId's-HEAD ' PvOCK-SEEIES.' 287 



stria tion parallel to the basal plane, often called the sahlite- 

 striation. Yery rarely there seem to be imperfect indications of 

 diallagic striation, in the form of a pinacoidal parting. The striated 

 and unstriated types are generally both present. The striated struc- 

 ture can also be detected, although to a less extent, in the more 

 ophitic types (see PI. XXXI, figs. 5 & 6). In this respect these 

 rocks differ from those described by Mr. Harker from Skye.^ I have 

 not been able to find here any corroboration of his suggestion that 

 this structure may possibly be connected with the depth at which 

 consolidation took place ; for it is by no means confined to the heart 

 of the intrusions. There is here good material for studying the 

 structure in all its forms ; and the conclusion seems to be necessary 

 that, although the schillerization, by which this structure is rendered 

 conspicuous, is a secondary feature, the structure itself is primary. 

 That a certain amount of schillerization is necessary to develop the 

 structure seems to be indicated by its frequent patchy occurrence 

 in the same crystal. Occasionally it is only exhibited along the 

 course of minute cracks and irregular fissures, where incipient 

 alteration might be expected to take place. That the structure is 

 not entirely secondary seems to be indicated by the occasional 

 occurrence of striated and unstriated augite intergrown together, 

 in which case the unstriated augite usually fringes the striated 

 form, and is, therefore, of later growth (see PI. XXXI, fig. 6). 

 Examples also occur in which twinned crystals, instead of exhibiting 

 the normal herring-bone structure, show the basal striation only 

 iiXDon one component. In other cases, an unstriated augite-crystal 

 includes a corroded fragment of the striated kind. There appears, 

 also, to be some relation between the occurrence of striated structure 

 and the development of enstatite. Thus crystallographic inter- 

 growths of augite and enstatite are common, but I have not observed 

 enstatite intergrown with a striated augite. This conclusion, 

 however, has only the support of negative evidence. 



Dr. Teall, in his description of this structure in the augite of the 

 Whin Sill, has suggested the possibility of its representing an 

 ultra-microscopic crystallographic intergrowth of rhombic and 

 monoclinic pyroxene, analogous to the perthitic structure of some 

 felspars. This view is to some extent supported by the observations 

 of Dr. A. Osann^ and Dr. A. H. Phillips,^ who conclude that it is 

 due to primary polysynthetic twinning. A similar origin has 

 been advocated by Prof. W. Wahl,"* who detected, in specimens 

 of diabase from Poglo (Aland), examples of augite in which the 

 striae, instead of continuing from edge to edge of the crystal, ter- 

 minate sharply in places against the prismatic cleavage. I have 

 noticed many such cases in the rocks now under discussion. I 

 have also found examples such as Prof. Wahl describes, in which a 



1 ' The Tertiary Igneous Rocka of Skye ' Mem. Geol. Surv. U.K. 1904, p. 110. 

 '^ Neues Jahrb. vol. i (1884) p. 45. 



3 Araer. Journ. Sci. ser. 4, vol. viii (1899) p. 267. 



4 'Die Enstatitaugitc ' Tschermak's Min. & Petr. Mitth. n. s. vol. xxvi (1907) 

 pp. 21, 26. 



Q. J. G.S. No. 254. u 



