270 C. A. Cotton — Block Mountains in New Zealand. 



had been almost or wholly destroyed prior to the deposition 

 of the overmass or cover. Though the later faulting to which 

 the existing relief is largely due appears to have followed the 

 lines of the older faults in some places, the displacement has 

 generally been reversed. 



Upon a planed surface of the undermass rests an overmass of 

 covering strata, which are preserved in the tectonic depressions, 

 but are almost entirely removed from the uplands and high- 

 lands. Over a great part of the area the overmass consists of 

 beds of fine quartz, sand, clay, brown coal, and greywacke 

 gravel. These are generally weak and incoherent with the 

 exception of an indurated layer — in places possibly more than 

 one layer — several feet in thickness of quartz grit with a sili- 

 ceous cement. Basalt, relatively a very resistant rock, occurs 

 towards the east interbedded with the covering strata over a 

 considerable area. 



The overmass of Central Otago could not have been continu- 

 ous unless an emergent land-mass outside this area furnished 

 the detritus. To the northeast, east, south, and west of Cen- 

 tral Otago, however, are remnants of a sheet of marine-cover- 

 ing strata, and to the north the undermass is composed almost 

 entirely of greywacke. The lower beds of the cover through- 

 out Otago are accumulations of detritus resulting from denuda- 

 tion of schist similar to that on which they rest. It is quite 

 probable, therefore, that some portion of the eroded surface of 

 the undermass has never been covered and so is a true pene- 

 plain (possibly with monadnocks). But the overmass was 

 much more extensive than now. It is significant that this con- 

 clusion, at which the writer arrived independently from a study 

 of the geomorphology, had been reached much earlier from a 

 study of the beds themselves by McKay, and clearly stated in 

 his later writings. 



McKay writes of the so-called quartz di'ifts (a term applied 

 in New Zealand to superficial and bedded auriferous deposits 

 of fine conglomerates or grits — not glacial deposits) as follows : 



"These accumulations are so disposed that they are in a large 

 measure — what remains of them — protected from being destroyed 

 by ordinary denuding agents, being either overlain by younger 

 /deposits or involved between older and younger strata, so that 

 the same result is effected. That their area in past times was 

 much greater than at present there is abundant evidence in the 

 disjointed scattered patches that are preserved and in the great 

 abundance of cement stones [" sarsen stones" of cemented quartz 

 grit] over surfaces considerably distant from any deposit of loose 

 quartz drift, and the quantities also of this particular kind of 

 rock in the newer drifts and recent gravel deposits in interior 

 Otago." (1897, pp. 88-89.) 



