NOTES ON A COLLECTION OE IGNEOUS ROCKS 

 EROM LORD HOWE ISLAND. 



Lifroduction. — The collection comprises about twenty specimens, collected 

 by Mr. R. Etheridge, junr., and Mr. Alexander Morton. 



Classification. — All the hand specimens examined belong to the basalt 

 group. They appear to belong to three leading types. — 



1. Basalt u-ith olivine. — These are chiefly dense dark greenish-gray 



rocks, for the most part little affected by decomposition, some 

 of them probably being of comparatively recent origin. 



2. Basalt witJiouf olivine, laterific. — This is a lateritic rock, of a dull 



brick-red colour, soft, earthy and amygdaloidal ; passing in places 

 into scoria. This lava may be partly submarine. 



3. Basalt, dlahasic. — A hard dense pyritous rock of a greenish-gray 



colour, and resembhng an andesitic dolerite ; it has undergone 

 extensive alteration, and is probably of considerable geological 

 antiquity. 



Detailed Description of Specimens. — Type 1. Basalt irifh olivine [Slide 

 jSTo. 11, from the " Grulch" North Cliff, is a dark-gray cellular basalt, having 

 its joints and steam-holes partly filled with calcite. 



On weathered surfaces the rock has a very fresh aspect, and has suffered 

 less, perhaps, from decomposition than any of the other specimens. It 

 outwardly resembles the vesicular basalt Ko. 6, or (30 a. and b.) north of 

 Ned's Beach, at the Point. 



Under the microscope the rock is seen to consist of a microcrystalline base 

 of magnetic iron and granular augite, enclosing micro-porphyritic grains of 

 olivine, occasional zeolites, and a good deal of glassy interstitial material. 



The felspars, chiefly triclinic, occur in minute lath-shaped crystals never 

 micro-porphyritic. 



Magnetite is abundant, though not present in sufficient quantity to make 

 the base opaque, as m the succeeding section. Ilmenite is also recognizable. 

 Augite is present, onlyin very minute grains of a ])ale purplish-brown colour. 



The olivine grains are in striking contrast to the rest of the rock con- 

 stituents. They are much decomposed and of very irregular shape and 

 uneven size, most of them having the appearance of fragments broken ofl" 

 larger grains. All the grains are surrounded by a zone of a reddish-brown 

 decomposition mineral, and the cracks traversing the grains are lined with 

 similar material. Green serpentinous matter is also observable towards the 

 centre of the olivine grains. 



The absence of any sign of decomposition or fracture in the rest of the 

 rock, as compared with the much decomposed and fractured state of the 

 olivine, argues a derivative origin for this mineral. 



Specimen No. 23 [Slide 2], exact locality unknown. — This is a dense basalt, 

 of a blackish-gray colour on weathered surfaces, very little decomposed, and 

 rendered slightly porphyritic by crystals of augite and grains of olivine. The 

 rock consists of a blackish-gray, rather opaque microcrystalline base of 

 triclinic felspar, and abundant magnetic iron, with porphyritic crystals of 

 augite, felspar, and olivine. 



The base contains so much magnetic iron as to be nearly opaque, excepting 

 at the thin edges of the section, and it appears to be wholly devitrified. 



