272 



THE GEOLOGIST. 



given off laterally by this main or dividing range are of the character 

 generally of ravines or gorges, occupied by mere mountain-streams ; the 

 "flats" or alluvial tracts at their mouths, and on the coast, are incon- 

 siderable. 



This mountain-range consists apparently of slates of Silurian age, 

 generally of argillaceous character, but greatly altered by contact with, or 

 proximity to, numerous outbursts or intrusions of trappean and other 

 rocks. The mountains are so densely wooded, and so difficult of access, 

 that it is only here and there in the gorges of the streams that sections of 

 these slates may be examined. In these sections the slates are frequently 

 found to resemble Lydian stone or the slaty varieties of basalt, such as 

 clinkstone ; while they are disposed more or less vertically, their irregular 

 upturned edges affording the most convenient and abundant " pockets " 

 for the detention and storage of the alluvial gold washed from the higher 

 grounds. 



Local geologists describe the fundamental rock of the Coromandel moun- 

 tain-system as granitic, and the granite as forming here and there the 

 " aiguilles " of the dividing ridge ; but Dr. Lindsay met with no granite in 

 situ, nor did he discover granitic boulders or pebbles in the boulder-cla} r s 

 of the auriferous drift, nor in the shingly beds of the mountain streams 

 about Coromandel Harbour. 



The Coromandel slates are characterized by their prominent and nume- 

 rous quartz "reefs," consisting of auriferous quartzites. Here and there, 

 where the dense vegetation admits, these reefs are met with in situ, fre- 

 quently as " dykes," standing prominently above the general level of the 

 slates ; sometimes forming the top of the dividing ridge itself. The proxi- 

 mity and abundance of such quartzites are sufficiently indicated by the 

 immense numbers of huge quartz-boulders or blocks which bestrew the 

 low ground, and occupy the ravines and gorges ; which blocks are charac- 

 terized by comparative angularity. The quartz is frequently of the porous, 

 light, spongy character so prevalent in the gold-fields of Australia, Nova 

 Scotia, California, and other auriferous countries ; and its colour is fre- 

 quently buff, brown, ochrey, or vermilion, the result, apparently, of differ- 

 ent degrees of ferruginous impregnation. 



The auriferous drift is mostly of the character of the newer or upper 

 Tertiary drifts of the Otago gold-fields, consisting essentially of — a. 

 variously-coloured clays ; b, boulder-clays, also variously coloured ; and 

 c, gravels, of the " chopped slate " character, the debris of the component 

 rocks of the parent ranges, which gravels rest immediately on the " bed- 

 rock " or slate. In this gravel, as at Otago, the gold chiefly occurs ; hence 

 to these gravels are, as yet, mainly directed the operations of the miner. 

 The gold itself occurs in the form of dust, scales, or nuggets — frequently 

 as scaly nuggets or " pepites," but still more generally dendritically dis- 

 seminated in quartz-pebbles, which are usually ochrey or brownish in 

 colour. It is largely associated with iserine (titaniferous iron-sand), ap- 

 parently of the character of that so abundant at Taranaki. This mineral, 

 indeed, appears to be associated with gold in almost all the New Zealand 

 gold-fields. 



The prevalent volcanic rocks, which burst through, overlie, or are other- 

 wise associated with the slates, are mainly various trachytes, tuffs, basalts, 

 and syenites. A hard breccia, consisting to a great extent of fragments of 

 jasper and flint, resembling somewhat the "cement" or quartz conglome- 

 rate of the older or lower Tertiary auriferous drifts of the Otago gold-fields, 

 occurs on Becson's Island, in Coromandel Harbour, which island is mainly 

 or altogether tufaceous. Boulders of basalt and syenite bestrew the tops 



