OF THE SOUTH OF THE ISLE OF MAN. 445 



the contours occur. Twiuniuj^ is common ; but perhaps the most 

 remarkable feature is tlie occurrence of irregular aggregates of 

 augite, wliich by ordinary light look like single crystals, but by 

 polarized light are seen to consist of perhaps half a dozen indi- 

 viduals, often twinned, with irregular boundaries where they meet. 

 The augite crystals are sometimes traversed by cracks occupied by 

 green fibres, lying at right angles to the cracks ; these fibres are 

 probably chlorite. Patches of magnetite or ilmenite occur. 



The matrix in which the abundant augite-crystals are embedded 

 is now a confused mass of green alteration-products, by which it is 

 hard to say what original minerals are replaced. 1 hesitate to give 

 a name to this rock ; but, as I believe olivine to have been originally 

 present, 1 propose to term it provisionally an augite-picrite-por- 

 phyrite. 



(f) The Olivine-dolerite Series. — I use the word " dolerite " as a 

 general term (as Kosenbusch uses "basalt") irrespective of the 

 coarseness of grain or compactness of the rock. 



The rocks are, when fresh, dark coloured, almost black, save 

 where the abundance of olivine gives them an olive tinge. They 

 weather green internally, and dark brown or sometimes ochreous at 

 the surface. 



As is usually the case, there are differences in coarseness of grain 

 according to the width of the dykes and the distance from the 

 salbands. A specimen from Grenea, near Strandhall, is most com- 

 pact, though pseudomorphs of porphyritic olivine can be made out 

 with a lens ; No. 81 (23-feet dyke at Strandhall) is the most coarsely 

 crystalline, but differs from the rest in appearance, being speckled 

 with white owing to the abundance of analcime. As a rule the 

 porphyritic olivines are the only minerals which can be made out. 

 They are best seen in some of the dykes at Langness. Small 

 amvgdules and pseudo-amygdules are common. The specific gravity 

 of No. 189 (3o-feet dyke north of the Goayr) = 2-90; JS'o. 192 

 (12-feet dyke south of Martha Gullet) = 2-92 ; No. 196 (2-feet- 

 dyke near the northern copper mine, Langness) = 2-89. 



Sections cut for the microscope give the following data : — 



Original Minerals. 



Olivine. 



Augite. 



Plagioclase. 



Magnetite. 



Pieotite. 



Apatite. 



Secondary Minerals, 



Serpentine. 

 Chlorite. 

 Calcite. 

 Magnetite. 

 Analcime, and 



perhaps other Zeolites. 



The structure is ophitic, and in many cases distinctly porphyritic, 

 the most porphyritic specimens being 189, 192, 190, &c. The por- 

 phyritic constituents are olivine, pieotite (included in the olivine), 

 and in some cases plagioclase. The rock is in most cases holocry- 

 Stalline, but possibly Nos. 81 and 166 (4-feet dyke at Strandhall) 

 may be exceptions, and the occurrence of amygdules, in some cases, 

 leads one to infer the original presence of glass. 



