XVIII OLIVINE-BASALTS 



259 



2'9i to 3-01. All the four species indicated by the varying length 

 of the felspar-lathes are represented in my collection. The rocks 

 of the first two, writh the felspars averaging less than 'i mm. and 

 between 'i and -2 mm., are compact aphanitic basalts, only 

 displaying an occasional small phenocryst of augite and blackish- 

 grey or bluish-black in colour. Those of the last two species, with 

 the average length of the felspar-lathes '2 — -3 mm. and -3 — -5 mm. 

 respectively, are lightish-grey coarse-textured rocks of the doleritic 

 type. In all the rocks no pyroxene phenocrysts are displayed 

 in the slide ; and the olivine phenocrysts, which are very numerous, 

 do not usually exceed i mm., though occasionally the average 

 size is I '3 mm., and not infrequently it is only "5 mm. In some 

 cases where the larger olivine phenocrysts lie athwart the current of 

 the felspar-lathes, the smaller {"'S mm.) lie with their long axes parallel 

 to the flow. The olivine is either fresh, or it may be beginning to 

 serpentinise in the cracks, or it may be in part hematised. The 

 crystals may have regular outlines, or they may be rounded and 

 sometimes deeply eroded. The pyroxene granules of the ground- 

 mass are of pale-brown augite, and average 'Oi to -03 mm. in size. 

 Occasionally a few prism forms occur, giving extinctions of 30° to 

 40°. In the case of the more compact rocks, with the felspar-lathes 

 averaging less than '2 mm. in length, lamellar twinning is but 

 scantily to be noticed in the lathes, which give extinctions measured 

 from the long axis of 20° to 25° and by the twin lamellae of 30° to 35°, 

 indicative of acid labradorite in both instances. With the coarser 

 doleritic grey basalts, where the felspar-lathes are stouter and have 

 an average length exceeding -2 mm., lamellar twinning is more 

 frequent ; the extinctions afforded by the lamellae range between 

 15° and 25° (medium andesine). Residual glass is scanty in these 

 rocks, and in the grey dolerites it is often difficult to recognise 

 any. 



2. Division of the Olivine-Basalts 



Characters. — Scanty olivine. 

 Formula. — Oliv, matr,pauc, 



25. Genus of the Olivine-Basalts 



ForivIULA. — Oliv, matr, pauc, non-flu, gran, phen, vitr. 



Characters. — Olivine scanty. Felspars (lathes and prisms) 

 of the groundmass not in flow-arrangement. Pyroxene of the 

 groundmass granular. Glassy plagioclase phenocrysts. 



S 2 



