Fox. — The Waitemata Series. 463 



the Waikato, I see no source from which pumice is likely to 

 have been derived by transport, and it does not occur in the 

 beds below or above the breccia. If the pumice is really part 

 of the ejected matter, we have a large range of lava from 

 basalts to rhyolites, indicating probably a long period of 

 volcanic activity. In the present state of our knowledge, or 

 lack of knowledge, of the Waitakerei lavas it is scarcely safe 

 to generalise, but it is interesting to note that no rhyolites 

 have yet been found in situ at the range, though andesites are 

 abundant and andesitic basalts still more common. It is 

 possible that the acid lavas rose in the vents but never flowed 

 out as lava. Dykes and necks are frequently more acid than 

 the lava-flows, and this seems here to be the case." 



I have drawn some sections of the lava as seen under 

 the microscope, but the drawings are merely diagram- 

 matic (Plates XXXIX. and XL.). The shaded portion 

 represents the ground-mass (which js usually opaque, but 

 sometimes consists of a brown glass). Owing to this fact the 

 phenocrysts appear in the diagrams to stand out from the 

 ground-mass more than they do m the rock. The shading, 

 moreover, is not quite true to nature, especially in fig. 1, 

 where the difference in shade between the two generations of 

 augite is accentuated. The mosaic of granules in fig. 2 is 

 only seen, of course, under polarised light. 



Fig. 1: St. Helier's Bay. — The ground-mass consists 

 largely of brown glass partly devitrified, and containing 

 numerous laths of feldspar and magnetite. The phenocrysts 

 are chiefly augite, in two generations. The smaller crystals 

 present irregular rounded outlines, and are yellowish-brown 

 in colour; the larger crystals present more regular six-sided 

 outlines, are dark-green, and contain inclusions, especially of 

 magnetite. The phenocrysts of feldspar frequently consist of 

 an outer shell, enclosing brown glass. Others show perfect 

 zonal structure. Striping is absent. There are a few 

 amvgdules. 



Fig. 2 : Deep Creek. — This is a highly amygdaloidal rock, 

 the secondary mineral, which in this case is chalcedony, not 

 only filling the vesicles but also replacing the phenocrysts, 

 none of which appear under the microscope. With polarised 

 light the pale amygdules break up into a mosaic of granules, 

 greys and yellows of the first order being the colours. Each 

 amygdule is bordered by a row of minute granules very regu- 

 larly arranged, the centre consisting of granules of a larger 

 size. Sometimes streams of small granules connect separate 

 amygdules. Feldspar laths are not numerous in the opaque 



* Sir A. Geikie : " Andent Volcanoes of Great Britain," vol. i 

 pp. 61, 62. 



