﻿602 MR. J. V. ELSDEN ON THE IGNEOUS ROCKS [Aug. IOO5, 



rather acid ^ligoclase. The twinning is normal in nearly all the 

 specimens examined, the pericline-type, although occasionally seen, 

 being uncommon. In a variety from Tresseysilt a somewhat un- 

 usual ' window-shaped' twinning occurs, in which four rather square 

 parallelograms have a nearly-parallel simultaneous extinction, and 

 are surrounded by a framework, extinguishing at 10° to its length. 

 In the 45° position all trace of the twin-structure disappears. 



The accurate determination of the felspars in many of the rocks 

 is rendered difficult by their turbidity. 



Secondary products. — Among secondary products sphene 

 and leucoxene are everywhere present, but call for no further 

 notice, with the exception of the marked tendency of the enstatite- 

 pseudomorphs to include little aggregations of sphene-granules. 



Epidote is very abundant in the diabases, but is not so notice- 

 able in the other types. Granular epidote often shows a marked 

 tendency to develop round the margins of chlorite-areas in the 

 gabbro- and diabase-types, and, beginning thus, gradually spreads 

 over the whole area. In quantity, it generally bears a direct 

 proportion to the amount of alteration exhibited by the felspars. 

 In the enstatite-bearing rocks, in which the felspars are of a more 

 acid type, this feature is not noticed, a character which agrees 

 with Prof. Zirkel's statement that saussuritization is not usually 

 conspicuous in the norites. 1 An interesting occurrence in several of 

 the latter is that of a colourless uniaxial mineral of tetragonal form, 

 with rather high double refraction, and rectangular cleavages. 

 The refractive index is higher than quartz. I refer this to one 

 of the scapolites. Prof. Judd has explained the conversion of 

 plagioclase-felspar into scapolite, 2 and it is a significant fact that 

 the rock of Oodegaarden, Norway, in which this conversion is 

 described, is a gabbro containing rhombic pyroxene. Similar 

 occurrences have been noted in the diabase-dykes of Lake Champ- 

 lain, 3 and in the diabase of the Pyrenees. 4 In Canada, also, 

 Prof. F. D. Adams and Prof. A. C. Lawson have noted the occur- 

 rence of scapolite in an enstatite-diorite. 5 In the present case, the 

 scapolite has been noted in a few examples only in the St. David's- 

 Head rocks ; it has all the appearance of being of secondary origin, 

 and not, as considered by Mr. J. E. Spurr, in the case of some 

 Alaskan rocks, a primary constituent. 6 



Of other secondary products chlorite is an abundant con- 

 stituent of the Strumble-Head diabases, but is less prominent in the 

 quartz-norites of the St. David's-Head district. In a few instances 

 vermicular chlorite (helminth) is noticeable in these varieties. In 

 the enstatite-diabases there is evidence of a gradual passage of 



1 ' Lehrbuch der Petrographie ' 2nd ed. vol. ii (1894) p. 776. 



2 Min. Mag. vol. viii (1888-89) p. 186. 



3 J. F. Kemp & V. F. Marsters, Bull. U.S. Geol. Surv. no. 107 (1893). 



4 A. Lacroix, Comptes Rendus Acad. Sci. Paris, vol. ex (1890) p. 1011. 



5 ' On some Canadian Rocks containing Scapolite, &c.' Canad, Rec. Sci. 

 vol. iii (1888-89) p. 185. 



c ' Scapolite-Rocks from Alaska ' Am. Journ. Sci. ser. 4, vol. x (1900) p. 310. 



