Shrewsbury. — O71 the Auckland Volcanoes. 375 



almost holocrystalline, owing to the pressure under which the 

 rock crystalhzed, and the comparatively long time it took to 

 cool. The olivine in this case (rock-section 72, PL XXXV), 

 is seen to be, for the most part, unaltered, but in places has 

 yielded ferruginous products. 



Besides the well-defined crystals (of which there are two in 

 this figure), there are small irregular patches of this mineral 

 (as at a) which with ordinary light can hardly be distinguished 

 from the augite. The latter occurs in fair-sized plates and 

 irregular crystals. 



With regard to the feldspar, there is a marked difference in 

 its mode of occurrence in typical sections of the two extreme 

 classes. In the first, or semi-vitreous, it is wholly in the form of 

 minute laths and bars mingled with the magnetite dust and 

 glassy matter (as in rock-section 70, PI. XXXV.) ; in the last 

 the crystals are larger and broader, and there is, in addition, a 

 development oi incites of this mineral, intercrystallized with the 

 other constituents more after the structure of the diorites and 

 plutonic rocks generally. The magnetite also occurs not as 

 fine dust, but as large crystals or plates. 



Between the types of rock of which these sections are taken 

 as illustrations a complete gradation can be traced, and the 

 transition followed from a semi-vibreous rock on the one hand 

 to the more holocrystalline state of the same or a similar 

 rock on the other. In other words, all these basalts, though 

 their appearance varies under the microscope, merely repre- 

 sent ditferently-cooled portions of one and the same molten 

 mass. The constituents are everywhere the same, the olivine, 

 magnetite, and glassy matter decreasing, and the feldspar 

 increasing, as we recede from the surface of cooling ; and no 

 particularly-glassy or particularly-crystalline variety charac- 

 terizes the lava from either of the hills. 



In speaking of the constituents, I have throughout referred 

 to the pyroxenic constituent under the general name of 

 " augite ; " and this appears to be the only form of pyroxene 

 present. I have examined the optical characters of this 

 mineral in a number of sections of the basalt under considera- 

 tion, and have been unable to detect any rhombic pyroxene : 

 in other words, the pyroxene apparently occurs only in the 

 monoclinic form. It remains to decide to which of the 

 monoclinic pyroxenes the mineral in question belongs. 

 Unfortunately the crystals are nearly all very irregular and 

 imperfectly developed, with no definite outlines or cleavages. 

 They are, in fact, rather grains or granules. Where, however, 

 individuals are sufficiently well developed they can be identified 

 as augite, showing the short prismatic forms characteristic of 

 that mineral. Long columnar forms, on the other hand, such 

 as are almost invariably assumed by diopside and acmite, 



