DEPOSITION ON CONTINENTAL SHELF AND SLOPE 157 
not, in general, be subjected to very prolonged chemical and 
mechanical disintegration on the shelf before being buried. When 
sandy it may contain, but little altered, grains of the less stable 
minerals of the shore rocks. If these rocks are predominantly 
igneous the topset sands when consolidated may form arkose or 
greywacke, similar to that forming the great bulk of the strata in 
the mountain ranges of New Zealand. Of these rocks, commonly 
ascribed to the Maitai System, Marshall writes: 
The material of which all these rocks is composed has been derived from 
plutonic masses, for they are composed of grains of quartz, feldspar, and horn- 
blende or augite..... The great thickness of the sediments shows that the 
area was one of deposition for a considerable time, though the general coarse- 
ness of the material shows that the deposition was relatively rapid, and took 
place on a coast-line. Presumably the coast-line fringed a large continental 
area, from the surface of which rivers carried large quantities of sand.! 
Preservation of the grains of decomposable minerals, however, 
cannot safely be taken as certain proof of topset origin of the beds 
in which they occur, for it is conceivable that, with a very abun- 
dant supply of terrigenous material during a period of little or no 
subsidence, similar material might be buried in foreset beds. 
Barrell? has indicated the origin of alternations of sandy and 
muddy layers of small thickness as a result of topset deposition. 
The sediment on the outer, deeper part of the continental shelf, 
near the maximum depth at which the bottom is ever stirred by 
wave action, is affected only by the waves produced by exception- 
ally severe storms, such as occur only once in a number of years. 
During the interval between two such storms an unsorted mixture 
of mud and fine sand accumulates. When a storm occurs, the 
gentle stirring of the bottom which it produces causes the finer 
particles of the superficial layer to go temporarily into suspension, 
the larger grains remaining as a layer of clean washed sand. After 
the storm, subsidence being continually in progress, another layer 
of sandy mud is laid down above the sand, and by the time the 
next great storm occurs the sand layer and the deeper part of the 
tP. Marshall, Geology of New Zealand (Wellington: Government Printer, 1912), 
pp. 184-85. 
2 J. Barrell, “Criteria for the Recognition of Ancient Delta Deposits,” Bull. Geol: 
Soc. Am., XXIII (1912), 428. 
