214 Transactions. — Geology. 



the south-east culminates in Castle Eock, 1,724 ft. in height. 

 From the top of this peak the southern prolongation may be 

 traced for several miles. Exactly how far south the dyke 

 reaches I cannot say, the only satisfaction derived from an 

 attempt to solve this question being that of spending a miser- 

 ably wet night in the bush. This much is, however, certain : 

 that the dyke extends in a fairly straight line for nine miles, 

 and may possibly reach to Sugar-loaf Hill, in the Waiwawa 

 Creek, a distance of fourteen miles from Kikowhakarere Bay. 



The life-history of Castle Eock, or Motutere, to use the 

 more euphonious native name, may be here briefly sketched. 

 It owes its birth to the same agency that covered the whole 

 of the Hauraki Peninsula area with its immense deposit of 

 igneous rocks — viz., the folding of the Palaeozoic slates in a 

 direction parallel to the protaxis of New Zealand. From 

 the concomitant line of weakness welled forth immense flows 

 of lava similar to, but lesser in degree than, those of the 

 Hawaiian Islands at the present day. After the deposition 

 of the Upper Eocene andesites a period of quiescence ensued, 

 to be broken in Miocene times by volcanic outbursts at 

 Coromandel, and on the east coast from Port Charles to 

 Whangapoua. That these outbursts were very diffei'ent in 

 character from preceding eruptions is clear from the nature 

 of the ejecta, and from the numerous dykes that everywhere 

 radiate from the foci of eruption. For both the breccias and 

 the dykes of this period indicate a superabundance of steam, 

 in the former case by the propulsion of dehris from a crater, 

 and in the latter case by the rending and Assuring of the 

 adjacent rock. 



In the case of the Castle Eock dyke, the centre of eruption 

 was probably to the west of Kikowhakarere Bay, for at the 

 south end of this bay occur true lava-flows of an identical 

 rock. Moreover, the coarser breccias of Beeson's Island, a 

 little to the south, are petrologically identical with the rock 

 of the dyke and of the lava-flow, thus pointing to a common 

 origin. 



In few places along its course does the dyke show greater 

 resistance to the weathering agents than the enclosing rock. 

 At Motutere itself the rock is seen to be horizontally columnar, 

 the columns being about 10 ft. in length and 1ft. to 2 ft. in 

 diameter. Additional evidence in favour of slow local cooling 

 is furnished by the superior hardness and porphyritic nature 

 of the rock at this place. 



In hand specimens the rock is grey, with large porphyritic 

 hornblende and feldspar crystals. The specific gravity is 

 about 2-6. Under the microscope the base is seen to be 

 feldsitic, apparently the result of devitrification. The feld- 

 spars are both plagioclase and orthoclase, the former 



