McKay. — On Diatoms at the Bay of Islands. 375 



General Conclusions. 

 The description of the mineral constituents of the rock 

 shows that it belongs to the andesite group. However, the 

 large amount of olivine present points to the fact that it ought 

 to be placed in the basic series, if the analysis did not show so 

 large a percentage of silica. This renders it necessary that it 

 shall be classed as an intermediate rock. Since the amount 

 of olivine is so large, while the augite is comparatively scarce, 

 it ought to be called an olivine-andesite rather than an augite- 

 andesite, and it would form a link between the olivine-basalts 

 and the augite-andesites. 



Art. LII. — On a Diatom Deposit near Pakaraka, Bay of 



Islands, Auckland. 



By A. McKay, F.G.S. 



[Read before the Wellington Philosophical Society, 3rd August, 1892.] 



Two years ago I collected samples of a diatomaceous deposit 

 from the bed of a small lake near Pakaraka, Bay of Islands. 

 On my return to Wellington Mr. Maskell kindly undertook the 

 examination of the samples, which were from the surface, and 

 a foot below the surface, of the deposit. 



Mr. Maskell found that in the sample taken a foot below 

 the surface there were forms that did not appear in the surface- 

 deposit, some of which he considered as belonging to extinct 

 forms only found in a fossil state. 



Subsequently I read a short paper on the subject, and in 

 that I expressed the opinion that the deposit was not, as sug- 

 gested by Mr. Maskell, in part the result of the breaking-up 

 of pre-existing beds of diatoms, from which were derived the 

 extinct forms found in the samples examined. 



I, on the contrary, favoured the supposition that from the 

 top to the bottom of the deposit the different species of diatoms 

 had lived and died where their remains are now found ; and I 

 suggested that were samples obtained from the lower or deeper 

 parts of the deposit other genera might be shown to appear in 

 and be confined to the lower part of the deposit. 



Through the kindness of the Hon. Mr. Williams, of Paka- 

 raka, I was again enabled to submit to Mr. Maskell samples 

 taken at intervals of a foot from the surface to the bottom 

 of the deposit, which proved to be 9ft. in thickness. 



Mr. Maskell examined five specimen-slides from each of the 

 ten samples taken at a foot apart, and he has favoured me 

 with the following summary of the results obtained : — 



