446 ME. B. HOBSON ON THE IGNEOUS K0CK3 



Olivine occurs as large, frequently idiomorphic, crystals, bounded 

 by the usual pinacoid and dome faces. It is almost completely 

 fresh in 189, 19:^, 196 ; it is more frequently partly fresh and partly 

 represented by serpentinous pseudomorphs, and is wliolly represented 

 by pseudomorphs in most cases. The olivine is sometimes corroded 

 by the groundmass ; this is best seen in 166, 189, 192, 196, &c. 

 Aggregates of olivine crystals frequently occur. 



The augite is violet-pink in colour by transmitted light, and is in 

 almost all cases distinctly ophitic. The ophitic structure varies 

 from the large coherent plates into the margins of which felspars 

 project in No. 81 to the, if I may so term it, trellis-like ophitic struc- 

 ture of the23-feet dyke at Knockrushen, where the augite is cut up 

 into small, usually triangular, sections by a trellis-like network of 

 felspars. 



The plagioclase occurs both as a porphyritic constituent and in 

 the groundmass. The porphyritic plagioclase gives lath-shaped 

 sections; it is present in 189, 192, 196, &c. In the other sections 

 only one generation of plagioclase, viz. that of the groundmass, 

 occurs ; this also gives lath-shaped sections. According to 

 Mr. Eutley (in Messrs. Dickson and Holland's paper *) the 

 plagioclase in three specimens, judging from the extinction-angles, 

 was an orthite, while in one specimen felspars which "may probably 

 be referred to bytownite " were also present. 



Magnetite (or ilmenite) is abundant, and appears to belong to the 

 groundmass. It also occurs as a secondary product in the olivine 

 pseudomorphs, e. g. in Nos. 81 and 189. 



The picotite (or chromite) occurs as yellowish-brown idiomorphic 

 crystals of small size included in the olivine, and is therefore an 

 intratelluric porphyritic constituent. The only section in which I 

 failed to observe it in the olivine or pseudomorphs is No. 81. 



Apatite in acicular crystals up to '5 mm. long is abundant in 

 No. 81, in which also pyrites occurs. 



Serpentine, as a product of alteration of the olivine, is abundant ; 

 it occupies irregular cracks, or completely replaces the crystal as a 

 pseudomorph with mesh-structure. 



Chlorite is present as an alteration-product of the augite. 



Calcite occupies amygdulcs and pseudo-amygdules. 



Analcime occupies the angular spaces between the felspars in Nos. 

 81 and 166, and either } artially or completely fills amygdules in 166. 

 It is generally isotropic, and often exhibits brownish granular 

 cloudy patches, as does the analcime in the dolerite of Salisbury 

 Craigs, Edinburgh f. 



Section 189 (35-fcet dyke N. of the Goayr, Langness) differs so 

 much from most of the other sections of olivine-dolerites as to 

 deserve special description. The most distinctive points about it 

 are the marked contrast between the fine-grained groundmass and 

 the porphyritic constituents. 



* Proc. L'pool Geol. Soc. vol. vi. pt. i. (1889) pp. 129, 130. 

 t See also Teall, ' Brit. Petrogr.' (1888) pi. xxii. lig. 1, Analcime-diabaee of Car 

 Craig. 



