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1918.] ' The N.Z. Journal of Science and Technology-. 283 
mainly marine. The sedimentary cover, largely a marine but partly a 
fluviatile deposit, lay (and in places still lies) upon a planed surface of an 
undermass of complex structure. 
The covering strata are, in general, relatively weak, consisting in great 
part of mudstones and incompletely indurated sandstones. . Somewhat 
more resistant than these are beds of limestone, and there are some masses 
of resistant cemented conglomerate, and in some districts lava sheets 
interbedded with the other covering strata ; but these hard rocks are 
relatively rare, and are weakened by being interbedded with the softer 
strata of the cover. 
In contrast with the covering beds the rocks of the undermass are 
generally highly resistant. In northern Nelson these are indurated shales 
or slates, quartzite, quartz schist, crystalline limestone, and intrusive 
granite, the limestone being the weakest member. In other parts of New 
Zealand by far the most common rocks are the indurated greywacke 
sandstones commonly ascribed to the Maitai system, except in western 
Otago, where gneissic and plutonic rocks occur, and in Central Otago, where 
the undermass consists entirely of schist containing abundant quartz, and 
forming a rock that is very resistant as compared with the unconsolidated 
sands and clays forming the cover in that district. 
In the deformation that produced the initial forms from which the 
main topographic features haye been carved there was a considerable 
development of strong warping as well as faulting, while in some places 
the covering strata were compressed -into folds. The uplifted blocks, in so far 
as they are not all bodily uplifted with fault-boundaries on all sides, are 
in part anticlinal, and the depressions are in part synclinal. The surfaces 
of these structural units or blocks in the initial stage were in part hori¬ 
zontal flat areas (high- or low-lying), while there were some flat back 
slopes or areas of surface with a gentle and nearly uniform slope, and some 
fold surfaces or areas of steeper slope not necessarily so uniform. The 
steep surfaces marking the boundaries between upland and lowland blocks 
were in some cases fault-scarps, but in other cases, where monoclinal folds 
replaced faults, they might be termed fold-scarps. A system of consequent 
drainage was established, and it is to this deformation of very modern date 
(geologically speaking) that the establishment of the majority of the New 
Zealand rivers way be assigned. Some rivers which now cross upland 
blocks in gorges that were obviously cut during the uplift of the blocks 
were possibly established on a newly emerged coastal plain just prior to 
the uplift, in which case they will now be strictly antecedent, or they 
may be consequent upon the relief produced by the first movements in 
the great deformation, but antecedent to the later movements, for all the 
blocks must not be thought of as moving at once. 
In the period during which the concourse of earth-blocks forming New 
Zealand has been subjected to subaerial erosion, the covering beds, except 
where they are particularly resistant, have been removed from the upland 
blocks; but they still survive on low-lying blocks, generally near the 
coasts, but in a few places inland in intermont basins (formed by low- 
lying surrounded by higher blocks). 
Where the cover is removed the fossil plain upon which it lay is either 
stripped and exposed as horizontal or gently tilted plateaux which are 
but little dissected, as in Otago, South Canterbury (fig. I), and northern 
Nelson ; or the undermass is maturely dissected, as, for example, over the 
greater part of the Rimutaka, Tararua, and Ruahine Ranges, or in the 
