436 Transactions. — Geology. 



two miles, at a distance varying from half a mile to a little 

 over a mile ; nor do they crop out on the ridge separating the 

 Waikawau Valley from the sea. These circumstances would 

 tend to show that the portions of the dyke-like masses now 

 exposed in the sea-cliffs and road-cuttings are the original 

 summits of igneous intrusions uncovered by comparatively 

 recent marine erosion. The contact-line between the slaty 

 shales and dykes is clearly exposed in a number of places, 

 but in all cases the degree of alteration of the clastic rock is 

 singularly little. In the immediate vicinity of each dyke the 

 slaty rocks are generally bent and shattered and the joints 

 much slickensided, as if the intrusions of the igneous mass 

 had exerted sufficient pressure to cause local thrust accom- 

 panied by shearing and displacement, more especially along 

 the planes of bedding. At the actual line of contact the 

 shales are merely hardened, or sometimes brecciated for a 

 depth of an inch or two. On many surfaces no alteration is 

 perceptible. 



In my memoir on the " Geology and Veins of the Hauraki 

 Goldfields "* I described these rocks as hypersthene-augite- 

 andesite, from the penological description and name supplied 

 by the late Professor Ulrich.f A subsequent visit to the 

 locality convinced me that an error had arisen, probably 

 through a misplacement or exchange of a label, and in January 

 of this year I made a further examination of these dykes, at 

 the same time collecting examples of each for more detailed 

 investigation. As a result of microscopic examination in thin 

 sections I find that these intrusive masses are composed of 

 hornblende-andesite. There is no evidence obtainable in the 

 field to fix the date of their eruption even approximately; but, 

 judging from the fact that they occur as dykes penetrating the 

 basement rock, and that no hornblende-andesite, so far as 

 ascertained at present, is known to occur associated with the 

 gold-bearing andesitic volcanic rocks which everywhere overlie 

 the slaty shales, it is perhaps only reasonable to infer that 

 they are at least older than the gold-bearing andesites. All 

 the dykes are much decomposed, making it difficult to obtain 

 good examples for microscopic study. 



Dyke No. 1. — This forms the first rocky bluff, some 

 30 chains south of the mouth of Waikawau River. It shows 

 an apparent width of about 240 yards. It is a dark-grey 

 compact rock ; feels somewhat rough to the touch. Hand 

 samples show conspicuous crystals of feldspar and hornblende, 

 the former up to 05 cm. and the latter 1 cm. long. In 

 polarised light the base is clear and crowded with feldspar 



* Trans. N.Z. Inst. Mining ngineers, 1897, vol. i., p. 81. 

 t I.e., p. 26. 



