The geology of the 15 Cook Islands in the south-central Pacific is briefly described and their geological history outlined. All are the summit portions of extinct Tertiary volcanoes; six of the seven Northern Group islands are atolls, four of the Southern Group are makatea-type islands, and the others include a high mountainous volcanic island, a hilly near-atoll, an atoll, and a sand cay. Radiometric ages from Rarotonga are Late Pliocene, foraminifera from the makatea (raised coral reef) of Mangaia are Oligocene or Miocene, and much of the limestone underlying the atolls is probably middle Tertiary.

Harzburgitic dunite mylonite, with lesser blastomylonite, o:1ivine schist, talcose ultramafic schist, and pegmatite, 'form a vertical septum 400-3000 ft wide, at least 14 miles long, in steeply folded 'and mylonitised gneiss on the Fiordland caast near Milford Saund. The mylanites are faliaceous and parphyroclastic, the schists xenoblastic granulitic to schistose. Contacts with adjaining gneiss are faults; that at Poisan Bay is the site of a hydragen-methane gas seep prabably generated by serpentinisation reactions.Associlated Lower Paleozoic rocks are medium to' maderately high grade (kyanite zone) gneisses of the amphibolite facies 0'1' metamorphism, naw extensively mylonitised. Mapped as Thursa, Saint Anne, and Milfard Farmatians, they formerly consisted 0'1' a basic igneous assemblage (gabbro, pyraxenite, etc.) probably assaciated with the uitramafites, as weH as greywacke, tuffaceaus sandstone, mudstone, and limestane. Minor basic dikes invaded the gneisses at a late stage of the Upper Palea:ZiOic regional metamorphism.The assemblage hlas undergone three distinct 'phases of metamorphism and deformation, as revealed by microtextures, petrafabric relationships, mesascopic fold-types and relationships, and associated neamineralisation. The first phase, carrelated with the wider regianal metamorphism of Fiardland, may have shortly followed or even accompanied the relatively anhydraus emplacement of ultramafites, strongly folded the hast gneisses, and develaped the ultramafic schists. Analyses by electron microprobe indicate thatalivine (Fa95) recrysivallised with little ,if lany significant change in campositian. A diffuse olivine subfabric in parphyroclasts and a strongly cleaved olivine polymorph may be relict fram this phase or from the earlier plutonic iemplacement. The second phase, of more strongly dynamic metamorphism occurred under lawer P-T canditions of the amphibo;Jite facies, generated common diaphtharetic minemls in the gneisses, and widespread but sparse antigorite in the ultramafhes, deV'eloped flexural-slip falds, mylanitic textures, foliation, and fluxion banding in all rocks. The third phase, more obviously relJated to' deformation at shallow depth, locally generated sparse antigorite, and in the gneisses quartz and mica, accentuated fluxion banding, ribbon-textures, land undulose extinction in minerals, and developed minar kink foilds and distinct zones of f'ault movement.The twa later phases are correlated with early movements of the nearby Alpine Fault; elsewhere alang the fault a wide range of shaUower cataclasites were developed after third phase movements.

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In the southern part of the South Island of New Zealand, the late Tertiary Kaikoura Orogeny was followed by severe glaciation of the western area of maximum uplift (9,000 ft), and by intense periglacial weathering in the interior range and basin district where differential uplift to 5,000 ft above sea level had occurred.Landforms of the interior now comprise smooth, broadly convex, fault-fold ranges of nearly horizontal schist ornamented with a profusion of shaft and pedestal tors, some of great size. The ranges are separated by broad synclinal and fault-angle valleys containing Tertiary quartzose freshwater deposits, and fluvioglacial alluvium that has been dissected into flights of terraces. The smooth summit profiles of the ranges were at first inherited from a Tertiary peneplain that was uplifted and exhumed during the Kaikoura Orogeny. The upland surfaces were sufficiently modified and degraded during the Pleistocene for most of the peneplain to have been entirely destroyed. The smooth inter-tor platforms of the high uplands, mantled only by thin solifluction debris, are therefore cryoplanation surfaces of mainly Last Glaciation age, only approximately parallel to the former stripped peneplain which was degraded at least 190 ft and possibly in places as much as 500 ft. The upper flanks of ranges are scalloped by a few, small, Last Glaciation nivation cirques and at lower altitudes are mantled by thicker solifluction deposits and by loess.Numerous smaller, irregular tors at low altitude are formed from salients of unweathered schist in a deep zone of Tertiary weathering immediately underlying tilted mid-Tertiary quartzose sediments, and their emergence is apparently continuing in the present cycle of erosion.Upland tors were formed during stages of cryoplanation, perhaps after intervening stages of interglacial weathering of the platform areas. Local structural factors in tot development are discussed. The age of the upland tors may be as little as Last Glaciation, but if cryoplanation was minimal they may have survived through one or more interglacial periods.