‘ DEPOSITION ON CONTINENTAL SHELF AND SLOPE 153 
among the sedimentary rocks known. to geologists, the former 
constituting shale and mudstone and graduating into marl, and 
the latter being represented by greensand and less pure glauconitic 
rocks. The samples obtained from the continental slope show well- 
marked stratification., As Murray remarks: ‘The analogues of 
the now-forming terrigenous deposits are to be found in all geologi- 
cal periods.’” 
In this connection an instructive comparison can be made of 
the chemical analysis of a composite sample of 4 “‘green muds” 
and 48 “blue muds”’ from various parts of the continental slopes 
with that of a composite sample of 78 shales taken as giving the 
average composition of the argillaceous sedimentary rocks, but 
probably not by any means all of foreset origin. The analyses are 
given by Clarke. 
STRUCTURE OF A SHELF BUILT DURING POSITIVE MOVEMENT 
Without going into the question of the possible causes of changes 
in the relative levels of sea and land the fact may be accepted that 
accumulation of sediment has in the past been very commonly 
accompanied by positive movement. No doubt positive move- 
ment is in some cases a rise in sea-level and in others a sinking of 
the floor upon which sediments are being laid down, and in still 
other cases a combination of both of these. It is convenient, how- 
ever, in an investigation of the probable structure of shelf deposits, 
to regard all changes in the relation of the shelf to sea-level as ver- 
tical movement of the shelf rather than of sea-level. The latter 
will affect the relation of the sea to the neighboring land also, that 
is to say, it will be regional in its effects, and obviously precisely 
similar effects will be brought about by regional subsidence. By 
regarding the sea-level as fixed and the shelf as subsiding, however, 
we are able to investigate the effects of differential as well as 
regional changes of level. 
To begin with, it may be supposed that a shelf of moderate 
dimensions has been built forward during a period of stillstand 
«Sir John Murray, The Ocean, p. 213. 2 [bid., p. 234. 
3 F. W. Clarke, ‘“‘The Data of Geochemistry,” U.S. Geol. Surv., Bull. 616, 1916, 
p. 514 and p. 28. 
