Bartrum. — Geology of Great Barrier Island. 119 



and other similar criteria observable at different portions of the coast- 

 lines in the mid- Auckland area ; for, as D. W. Johnson shows in Shore 

 Processes and Shore-line Development, the disturbance of the equilibrium 

 of the graded off-shore profile by uplift is the most general cause of the 

 building of these off-shore bars. 



In corresponding manner, at the Great Barrier Island, a graded profile 

 must have been established fairly early by vigorous wave-attack upon the 

 easily removed areas of low relief on the eastern coast, and barrier beaches 

 would soon come into existence upon any subsequent uplift taking place. 



Summary of Stratigraphy. 



Great Barrier Island is constituted by a basement mass of folded sedi- 

 ments, largely shales and greywackes, and herein called the " oldermass,"* 

 which extend over the northern part of the island as far south approximately 

 as a line drawn from the head of Katherine Bay on the west coast to 

 Tupawai on the east, and which are again exposed in a small area near 

 Harataonga Bay, farther south. These rocks have been extensively eroded 

 and then covered in the Tertiary by a sheet of andesitic volcanics which is 

 probably well over 1,000 ft. in depth. These are in turn overlain by later 

 acid volcanic rocks, with accompanying sinters, in a central area around 

 and south of Mount Hobson. 



The andesitic rocks are largely coarse fragmentals, with subsidiary 

 lavas ; they form the mass of the island south of the northern sedimentary 

 area, and are covered at higher levels by acidic rocks in the area mentioned. 



The Basement Sediments, or Oldermass. 



The writer closely examined the outcrops of the older sediments for 

 fossils, but was unable to find any, in spite of the fact that (fide Sollas and 

 McKay, 1905, vol. 1, p. 146) Hutton discovered a coral. There is little 

 doubt that the oldermass of the island is comprised of rocks substantially 

 the same as those of Coromandel Peninsula which yielded a few Mesozoic 

 fossils south of Coromandel (Fraser and Adams, 1907, pp. 49-50). In 

 facies they are mainly shales, but with moderately frequent greywackes 

 which are sometimes — as, for example, at Harataonga Bay — finely inter- 

 banded with the shales. In the same locality, further, a small amount of 

 fine conglomerate is displayed, which recalls somewhat the conglomerate 

 of the comparable Manaia series of Coromandel Peninsula (Fraser and 

 Adams, 1907, pp. 48-62). In the headwaters of Mine Bay Creek there is 

 a coarse conglomerate with greywacke boulders. 



One of the most striking features of the oldermass is the way in which 

 its rocks have been seamed by the numerous dykes mentioned in the 

 introduction above. A detailed account of their petrography will be given 

 in a later section 



A most interesting and important discovery was made of bands of 

 coarse conglomerate with abundant granitic, pegmatitic, and granulitie 

 pebbles and boulders. Undoubtedly these yield very definite information 

 as to the character of the earlier (pre-Mesozoic) land-mass affording the 

 clastic material. 



* A usage introduced to New Zealand geology by Cotton (1910). 



