1919.] 
Reviews and Abstracts. 
59 
REVIEWS AND ABSTRACTS. 
Conditions of Deposition on the Continental Shelf and Slope, by 
C. A. Cotton. Journal of Geology , vol. 26, pp. 135-60, 1918. 
Following his studies of various forms of land-surfaces and of coasts, 
Dr. Cotton, in the present article, considers the processes leading to the 
formation of the comparatively wide shelves which extend down to a 
depth of 100 fathoms and surround most of the large land-masses. In 
so doing he throws light on the mode of origin of certain marine strata in 
New Zealand. He regards the formation of the continental shelf as usually 
due in a minor degree only to the action of marine erosion, but as chiefly 
a constructional feature, being formed partly of river-borne material, partly 
from the detritus from coastal erosion, built out from the coast-line like 
confluent deltas. The material is conveyed from the coast across the wide 
gently sloping upper surface by the agitation brought about by the surface 
waves, and where deposited finally on this surface it forms the upper, or 
“ top-set,” beds. At a depth of 100 fathoms the slope changes rather sharply, 
and the terrigenous material which is conveyed beyond this depth comes 
to rest on the steeper outer surface or continental slope of the “ continental 
delta,” and forms the “ fore-set beds," which are of finer-grain size than 
the deposits on the continental shelf, and may be more or less mingled 
with terrigenous material, which has settled from suspension in the 
sea-water, as well as with other pelagic material of organic origin. The 
“ bottom-set beds,” which are deposited beyond the continental slope, 
consist entirely of pelagic material—terrigenous, volcanic, meteoric, or 
organic. The constancy of the presence of the continental shelf, and the 
uniform depth of the outer edge, indicate that the present-day shelf 
has taken form since the sea and land assumed their present relative 
levels, and therefore that the moulding of the shelf is geologically a very 
recent event. 
Where the crust has been stationary the fore-set beds are the most 
important, but with a constantly subsiding coast a larger thickness of more 
coarsely granular top-set beds may form, providing the supply of detritus 
be sufficient. In regions where such subsidence was intermittent (and these 
are very common) there may be also an associated variation in the level 
of the land, with a concomitant variation in the character of the terrestrial 
detritus, the distribution of top-set and fore-set beds, and the extent of 
the intermingling of pelagic and partly organic sediments. The “ Maitai 
system ” of New Zealand, consisting largely of greywacke, probably 
originated as a continental delta built out on a subsiding area from a land- 
mass largely composed of igneous rocks. At and beyond the edge of the 
continental shelf fine terrigenous muds may be intercalated with purely 
pelagic and organic deposits (limestones), and to this mode of origin 
is referable the Arnuri limestone of North Canterbury, lying between 
mud-stones and greensands, and perhaps also the Oamaru limestone of 
Otago. 
In regions of coastal uplift the top-set beds are thin, and they are largely 
removed, and on the continental slope there is a continuous and heavy 
sedimentation of material which may be somewhat coarse in texture. 
W. N. B. 
