82 DR. A. HOLMES OX THE PRE- CAMBRIAN [vol. lxxiv, 



(2) Picrite. 



The only picrite encountered in the country was that of the dyke 

 at the Ampwihi Crossing (fig. 4, p. 38). This rock (No. 20 J) 

 has already been described in detail by the present writer, and 

 analysed chemically by Dr. H. F. Harwoocl. 1 The following 

 particulars are quoted from the paper here cited : — 



' The specimens collected were from the margin — evidently chilled — of the 

 transverse dyke, and have a dark-grey colour, mottled with nearly black 

 glassy phenocrysts of olivine. Here and there are minute white amygdales, 

 the infilling consisting of an isotropic material that is probably glass. The 

 weathered surface is creamy-grey in colour, with rusty patches corresponding- 

 to the phenocrysts. The average specific gravity of three fragments of the 

 fresh rock is 3 - 08. 



' In thin section the rock is found to consist of corroded phenocrysts of 

 olivine in a fine-grained ground-mass composed mainly of elongated grains 

 of augite and enstatite, the former alternating with and sometimes inter- 

 grown with laths of soda-lime felspar. In places interstitial patches of pale 

 brownish-grey glass appear, and where a minute amygdale is seen it is found 

 to be composed of the same obscure material. (P. 151.) 



' In order to determine the specific gravities of the various minerals, a dif- 

 fusion column of methylene iodide and methylene iodide diluted with benzine 

 was prepared so as to give a range from 3*3 to 2'3. Olivine and magnetite 

 sank. A well-marked band of pyroxene formed between 3 - l and 3 - 2. Another 

 band formed at 2*73 (labradorite), tailing out above and below owing to the 

 difficulty of obtaining a clean separation from such fine-grained material. 

 Finally, another layer formed at the level corresponding to 2*50. 



' The results obtained are as follows : — 



1vr . , Percentage by Specific 



Mineral. Weight. " Gravity. 



Glass 8 2-50 



Labradorite, An T0 17 2'73 



Pyroxene 45 '3 1 15 (average) 



Olivine 29 3*45(?) 



Magnetite 1 5*17 (?) ' 



Average 3' 20 

 Total 100 (P. 152.) 



IX. Heavy Residues erom Crushed Rocks and 

 River-Deposits. 



Accessory minerals obtained from a number of gneisses and 

 granites by crushing and panning have already been described. 

 The details are here repeated in tabular form, for comparison with 

 the minerals of the heavy residues separated from river-sands and 

 gravels. A large number of heavy residues were examined in the 

 field during the course of prospecting work for gold and cassiterite, 

 and many of these were preserved for more detailed examination 

 in the laboratory. 



The most interesting result is the discovery of monazite 



1 A. Holmes & H. F. Harwood, Geol. Mag. dec. 6, vol. iv (1917) pp. 150-57 

 A: pi. xi. 



