14 E. ©. ANDREWS. 
THE PRESENT STRUCTURE OF THE PACIFIC. 
In this contribution to the growing literature of coral 
reefs it would be advisable, perhaps, to consider the general 
framework of the Pacific Region, together with brief 
references to the West Indies and to the Indian Ocean 
before considering the origin of coral reefs themselves. 
Geographical Unity of the Pacific. 
A geographical unit may be defined as a portion of the 
earth’s crust in which the formation of the major topo- 
graphical features has been controlled by one agency. This 
agency may be considered as operating ee within 
and around the whole area. 
Assuming for the present that the Pacific Region isa 
geographica! unit, the controlling agency in its formation 
appears to have asserted itself periodically from early 
geologic time, the successive periods of which, when con- 
sidered together, are characterised by the production of 
varied but related forms. These were similar in general 
shape and were situated similarly with respect to a great 
central feature, but the quantitative effect of the periodic 
control mentioned varied during the successive periods of 
its application. 
The controlling agency which determined the geo- 
graphical unity of the region under consideration appears 
to be related in some way to the sagging of the great sub- 
oceanic mass under its own weight and with the consequent 
“‘creep,’’ by undulations, of the bordering continents towards 
the low-lying block constituting the Hastern or Main 
Pacific. 
Evidence of Unity. 
Adverting to the consideration of the statement above as 
to the geographical unity of the region under discussion, it 
“will be found advisable to consider a few of the main features 
