British Association^ Edinburgh Meeting — Lapioorth. 231 
facts, and then, reading them in conjunction with what we actually 
know of the physical geography of the present daj', try to ascertain how 
such mutual agreement as we can discover may serve to aid the strati- 
graphical geologist in his interpretation of the true meaning of the geolog- 
ical formations themselves. We may not hope for many years to come 
to read the whole of this geological chapter, but we may perhaps mod- 
estly essay an interpretation of one or two of the opening paragraphs. 
In the physical geography of the present day we find the exterior of 
our terraqueous globe divided between the two elements land and water. 
We know that the solid geological formations exist everywhere beneath 
the visible surface of the lands, but of their existence under the present 
ocean floor we have as yet no absolute certainty. We know both the 
form of the surface and the composition of the outer layers of the conti- 
nental parts of the lithosphere; we only know as yet even in outline the 
form of the surface of its oceanic portions. The surface of each of our 
great continental masses of land resembles that of a long and broad 
arch-like form, of which we see the simplest type in the New World. 
The surface of the North American arch is sagged downwards in the 
middle into a central depression which lies between two long marginal 
plateaux, and these plateaux are finally crowned by the wrinkled crests 
which form its modern mountain systems. The surface of each of our 
ocean floors exactly resembles that of a continent turned upside down. 
Taking the Atlantic as our simplest type, we may say that the surface of 
an ocean basin resembles that of a mighty trough or syncline,buckledup 
more or less centrally into a medial ridge, which is bounded by two long 
and deep marginal hollows, in the cores of which still deeper grooves 
sink to the profoundest depths. This complementary relationship de- 
scends even to the minor features of the two. Where the great conti- 
nental sag sinks below the ocean level, we have our gulfs and our Med- 
iterraneans, seen in our type continent as the Mexican gulf and Hudson 
bay. Where the central oceanic buckle attains the water line, we have 
our oceanic islands, seen in our type ocean as St. Helena and the Azores. 
Although these apparent crust-waves are neither equal in size nor sym- 
metrical in form, this complementary relationship between them is al- 
ways discernible. The broad Pacific depression seems to answer to the 
broad elevation of the Old World — the narrow trough of the Atlantic to 
the narrow continent of America. 
Every primary wave of the earth's surface is broken up into minor 
waves,in each of which the ridge and its complementary trough are always 
recognizable. The compound ridge of the Alps answers to the compound 
Mediterranean trough; the continuous western mountain chains of the 
Americas to the continuous hollow of the eastern x'acific which bounds 
them; the sweep of the crest of the Himalaya to the curve of the Indo- 
Gangetic depression. Even where the surface waves of the lithosphere 
lie more or less buried beneath the waters of the ocean and the seas, the 
same rule always obtains. The island chains of the Antilles answer to 
the several Caribbean abysses, those of the ^Egean archipelago answer 
to the Levantine deeps. 
