1082 NOTES AND EXHIBITS. 



occurrence of well-crystallized magnetite enclosed in the olivine 

 grains and augite crystals, magnetite in this form being wholly 

 absent from the glass, as already stated. If tlie absence of 

 magnetite from the glassy base is to be accounted for by the very 

 high temperature of the lava at the time of its eruption keeping 

 the iron combined with the silica until the whole was suddenly 

 cooled, it follows that the olivine must have crystallized originally 

 at a temperature sufficiently low to allow of the crystallizing out 

 of the magnetite. The temperature of the basalt glass at the time 

 of eruption was certainly sufficient to fuse or dissolve the olivine, as 

 evidenced by the deeply eroded edges of the latter. The crystal- 

 lized magnetite in the olivine and augite crystals must therefore 

 have been formed at some time previous to the eruption which 

 produced the lapilli, or at any rate previous to the glassy lavas 

 reaching the surface, and its presence in the olivine, and in the 

 few larger crystals of augite, shows that these minerals must also 

 have crystallized previous to the out-pouring of the lava, and 

 consequently these crystals must have been formed at great depths. 

 Magnetite occui's in two distinct forms each belonging to different 

 dates. Firstly in well-defined octahedi'al crystals, and secondly as 

 fine dust forming an opaque dusty-brown zone round some of the 

 felspar crystals, and occasionally in dendritic forms in gas-pores in 

 the olivine grains and in the cracks in the glassy base. Most of 

 the magnetite is found crystallized in the manner first-described, 

 formina; enclosures in the olivine and in a few of the augite 

 crystals. 



Sumviary. — The microscopic structure of these lapilli, as roughly 

 sketched in these notes, warrants no more than provisional 

 inferences as to their possible history. Previous to the newer 

 eruption which produced the lapilli, olivine and augite along with 

 magnetite must have crystallized out in deep-seated underlying 

 regions, which may have formed the sources of one of the previous 

 I'asaltic eruptions. (There is a distinct evidence in this district of 

 at least two eruptions of basalt at widely different dates, the 

 lapilli belonging to one of the newer eruptions). The temperature 

 at which the olivine and augite crystallized may have been low 



