1^0 PROF. T. T. GROOM ON THE IGNEOUS ROCKS [Feb. I9OI, 



from the groundmass, the ingredients of which have suffered 

 similarly. 



Of the two varieties of the rock M 112, the specific gravity of 

 the darker is 2-67, and that of the reddish variety 2*62. Timins's 

 analyses (XIX & XX) of these two varieties gave as silica-yjer- 

 centages 52-12 and 56*03 respectively.^ Two other analyses (XVII "^ 

 & XX ^), which probably belong to intrusives in the Sandstone, 

 show 54*01 and 48*25 per cent, of silica respectively. If we may 

 rely on these analyses, it follows that the chemical composition of 

 the dykes includes both basic and sub-basic varieties. 



Rocks essentially similar to those intrusive in the HoUybush 

 Sandstone are associated with the Upper Cambrian Block and 

 Grey Shales (M 101«, 104, 117, 125, 157.T, 160, 183, & 209). 

 They take the form of circular bosses, or of small laccnlites or 

 sheets intercalated in the Shales. All the larger bosses in the 

 Shales belong to this series. The sills in the Black Shales have 

 sometimes bleached the latter. Possibly the sills are in some cases 

 connected with the circular bosses, but in no single instance is 

 the exposure sufficient to show the relation of the bosses either to 

 the sills or to the adjacent Shales.* The bosses possibly mark the 

 position of the pipes which supplied the material for the sills and 

 laccolites, or even for lavas poured out at the surface, and now 

 removed by denudation. 



Where best preserved, the material of these intrusive masses 

 shows a purplish-grey (M 101a) or a bluish-grey coloration (M 104, 

 183,209). More weathered examples (M117) become greenish- 

 grey, or dark-green (M 157a, l*iOa) ; and the most weathered 

 assume a red dish -yellow (M J04), or an orange colour (M 160). 

 The specific gravity of six of the rocks ranges from 2 58 to 2*67 

 (see Table I, p. 164). 



Microsco])ically th(^se rocks, for the most part, agree closely with 

 their representatives in the Hollybush Sandstone (PI. VII, fig, 5, and 

 text-fig. 5, p. 1 65). The groundmass is nearly in an unaltered condi- 

 tion, the original minerals being invariably more or less decomposed, 

 and replaced by serpentine, caldte, epidote, titanite, etc. The pheno- 

 crysts always include one or more of the following, or pseudomorphs 

 after them : — amphibole, augite, felspar, and ilmenite (see Table I, 

 p. 164). 



In all the rocks described above, among the secondary products 



^ Quart, Journ. Geol. Soc. vol. xxiii (1867) p. 355. ^ Loc. cit. 



^ Ediub. New Phil. Journ. ser. 2, vol. xv (18()2) p. 3. Other analyses in 

 the two papers by tliis investigator are stated to relate to igneous rocks in the 

 Hollybush Sandstone. Of these, No. XVIII. in the Quaiteily Journal almost 

 certainly refers to a rock in the Upper Cambrian Shales (see p. 17B) ; No. XVI. 

 in the same Journal may refer to a grey variety of the Hollybush Sandstone, 

 or to the h^ng dyke on Eaggedstone Hill. No. XXI. in ihe Edinburgh Phil. 

 Journ. possibly relates to the green Hollybush Sandstone itself. 



* The Shales immediately north-west of M 183 [see Quart. Journ. Geol. Soo. 

 vol. Iv (1899) pi. X'li] dip beneath the straight side of the semicircular boss, 

 but the actual junction is concealed. 



