in water would separate finer material from the coarser to produce tuffaceous and conglomeratic facies respectively. The phenomenon of volcanic mudflows satisfactorily explains the frequent occurrence of compact bedded accumulations of volcanic blocks and other debris at low altitudes in places some miles distant from the steeper flanks of the parent volcanoes For example, mudflows were observed by Perret (1924, p. 102) as post-eruptive features at Vesuvius in 1906: “The ash, sand and boulders were in no case … carried (by hot avalanches) more than 3 or 4 kilometres from the volcanic axis, and they remained consequently on the slopes of the mountain. Rain or snow would then … soak the porous mass until a certain consistency was reached, which conferred a degree of mobility differing widely in character from that of the hot avalanche but nevertheless permitting rapid descent through gullies and other natural depressions as a torrent of liquid mud. This carried blocks and boulders of all sizes” and “the mudflows massed their heterogeneous contents into conglomerates”. Fenner (1937) described a similar cold lahar at Katmai and particularly noted the resulting abrasion, for “the pounding and grinding undergone by the boulders seem to have been equivalent in their rounding effect to those produced on the ordinary boulders of river channels through hundreds of years of stream action” The larger andesite blocks exposed in the conglomerates of the Manukau Breccia are not as rounded as the smaller boulders, for usually only their corners and ridges are rounded off. Blackwelder (1928) believes from similar observations that this is due to the tendency for larger blocks more or less to float in a mudflow or even lie on its surface. The conglomerates of the Manukau Breccia, it is believed, were formed by processes similar to those outlined above, but we have yet to consider the mode of formation of the deposits of breccia that so commonly lie interstratified in lenses with the volcanic conglomerates. Are these breccias simply the ejecta of major explosions? Are they deposits of short-lived mudflows? Or are they the farthest travelled portions of block lavas of either the block and ash type associated with weak Pelean eruptions (Perret, 1935) or with the normal type (Washington, 1926)? The latter writer has described the filling of a lagoon by flow of non-vesicular blocks lacking pumiceous or scoriaceous material, and Williams (1932) traced a similar flow, consisting of andesitic basalt, over an area of 20 square miles. The writer believes that block flow of fragments, comparable to that described by Washington, partly explains many of the breccia deposits in the volcanic fragmental accumulations along the Waitakere Coast. However, some of the breccias may have been deposited directly from explosive activity and soon covered, before they could be sorted or rounded, by lahar material, for coarser debris left on slopes, with added ash, would provide a suitable source of later bulky mudflows. There is no clear evidence as to the type of volcanic activity which made available such vast amounts of fragmental material since details of the vents are lacking and most of the debris appears marine-bedded. The association of normal lava flows, scoriaceous debris, agglomerates, pumice shreds and accessory blocks around the vents at White's Bay, O'Neill's Bay, Anawhata, and that just south of Bethell's Beach points to eruptions of Vulcanian type There arises also, however, the possibility of mixed activity. For example there may have been phreatomagmatic explosions caused by seawater entering fissures opened by volcanic heavings; and, as already mentioned, some of the eruptions may have been of