DIASTROPHISM AND THE FORMATIVE PROCESSES 527 
is an inheritance of great moment. The work of regrading in the 
later period is relatively light and the later sea-transgression much 
facilitated. Even when the deformation between the periods is 
strongly felt within the same continent, it is usually confined to 
portions of the transgressive plane only and the rest remains avail- 
able for the next sea advance. For example, the Mid-Ordovician 
transgression formed a broad shelf covering half America; this was 
only partially warped on its east border by the Taconic folding, and 
the transgression of the Mid-Silurian was greatly facilitated by its 
inheritance from the Ordovician. The Ordovician had received a 
great inheritance from the Cambrian shelf-work. The work of all 
these was handed on to still later periods. This leads us to note an 
additional characteristic. 
8. The work of shelf-seas was cumulative. This is true also of 
base-planes, the working copartners of shelf-seas, but not in an equal 
degree. An uncovered base-plane is constantly affected by dis- 
integration and on any upwarp to destructive erosion against 
which it has no protection. The shelf-sea work is constructive and 
on any upwarp is defended by its own deposits. While it suffers, it 
has resources of resistance. In addition to this, its attitude is 
conservative and some notable uplift is necessary to expose all its 
strata to removal. It thus follows that a great sea-shelf once 
formed usually transmits its results to successive periods, and each 
of these adds its own extensions. There is thus developed a suc- 
cession of sea-transgressions recorded by terrane spread upon 
terrane, making up a series which is continuous until one of the 
greater diastrophisms cuts the cumulative process short. 
9. The upper faces of the continents are largely the products of the 
cumulative work of the shelf-seas, though much mutilated by dtastro- 
phism and erosion. It seems safe to say that 80 or go per cent of the 
surface areas of the continents bear some evidence of former shelf- 
sea work and 70 per cent or more are still mantled by the shelf 
products, though some notable part of this mantle has lost its 
original flat attitude. The relatively plane upper face of the con- 
tinents is to be assigned largely to base-planing and shelf-sea work 
in which the latter has left the more lasting, if not the greater, 
product. Shelf-sea work is pre-eminently a process of terracing. 
