REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



135 



Fig. 24. — Nepheline rock with acmite from Greenland Harbour- 



The ground-mass is entirely composed of nepheline, numerous 

 hexagonal or quadratic sections of which appear in the figure. 

 In this there are greenish lamellar sections of acmite. ^ 5 

 crossed nicols. 



extinction. Almost all the sections extinguish parallel to their length, and in the case 

 of an oblique extinction it never exceeds 

 3° or 4°. We consider this mineral to be 

 acmite, the presence of which has been 

 ascertained in rocks analogous to those 

 now described. The outlines of the pris- 

 matic zone are fairly clear, but the crystals 

 are corroded, and almost fibrous at the 

 extremities. Terminal faces are never seen, 

 except a rather low dome which is very 

 rare. The pleochroism, as shown by these 

 crystals, is dark green for rays vibrating 

 parallel to c, and yellowish for those per- 

 pendicular to that direction (see fig. 24). 

 Like the rocks encircling the creek, this 

 nephelinic mass contains numerous patches of fibro-radiated zeolites. 



A specimen taken at the contact of the phonolite and the encasing basalt shows 

 both rocks in juxtaposition, but quite distinct from each other. There is no gradual 

 transition, but a sudden passage from one to the other : on one side the reddish almost 

 spongy basalt, on the other the greenish grey compact phonolite. The latter is 

 brecciated, as if the eruption of the basalt had produced a friction-breccia. The 

 specimens of basalt taken at the contact are in some cases black compact tufas con- 

 taining lapilli, which are identical in structure and mineralogical constitution with the 

 basalt of Greenland Harbour. Fragments of phonolite are also seen, and sometimes 

 vitreous lapilli altered into palagonite. Amongst the fragments of minerals in this 

 tufa, olivine, augite, triclinic felspars, and large broken crystals of sanidine may be 

 seen. Some of these, especially the plagic- 

 clases, are entirely penetrated by silica, 

 which has converted them into pseudo- 

 morphs. A group of triclinic felspars is 

 here figured (fig. 25), which shows that 

 they are replaced in the upper part by 

 opal, in the lower by chalcedony. The 

 mass uniting the clastic elements of this 

 tufa seems to be of a vitreous nature, but 

 its characters are vague, and veiled by in- 

 numerable opaque grains, most probably of magnetite, which are scattered throughout 

 the substance. The phonolite part of this specimen which is joined to the basalt does 

 not present, from the point of view of micro-structure, anything to distinguish it from 



FlG. 25. — Basalt in contact with phonolite from 

 Greenland Harbour. 



Group of plagioclase epigeuised into opal on the upper part, 

 transformed into chalcedony on the lower. 5*5 crossed nicols. 

 Polarised light. 



