420 Notices of Memoirs — Prof. LapivorW s Address. 



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 this 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 our 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 each 

 ocean basin resembles that of a mighty trough or syncline, buckled up 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 descends ev^en to the minor features of the two. 



Where the great continental sag sinks below the ocean level we have our gulfs 

 and our Mediterranean, 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 symmetrical in 

 form, this complementary relationship between them is always 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 recognisable. 

 Tlie compound ridge of the Alps answers to the compound Mediterranean trough ; 

 the continuous western mountain chain of the Americas to the continuous hollow 

 of the Eastern Pacific 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 iEgean Archipelago answer to the 

 Levantine deeps. 



Draw a section of the surface of the lithosphere along a great circle in any 

 direction, the rule remains the same : crest and trough, height and hollow, succeed 

 each other in endless sequence, of every gradation of size, of every degree of 

 complexity. Sometimes the ridges are continental, like those of the Americas ; 

 sometimes orographic, like those of the Himalaya ; sometimes they are local, like 

 those of the English Weald. But so long as we do not descend to minor details 

 we find that every line drawn across the earth's surface at the present day rises and 

 falls like the imaginary line drawn across the surface of the waves of the ocean. 

 No rise of that line occurs without its complementary depression ; the two always 

 go together, and must of necessity be considered together. Each pair constitutes 

 one of those geographical units of form of which every continuous direct line 

 carried over the surface of the lithosphere of our globe is made up. This unit is 

 always made up of an arch-like rise and a trough-like depression which shade into 

 each other along a middle line of contrary curvature. It resembles the letter S, or 

 Hogarth's line of beauty, and is clearly identical in form with the typical wave of 

 the physicist. Here, then, we reach a very simple and natural conclusion, viz., 

 the surface of the earth-crust of the present day resembles that of a series of crust- 

 waves of different lengths and different amplitudes, more or less irregular and 

 complex, it is true, but everywhere alternately rising and falling in symmetrical 

 pairs like the waves of the sea. 



Now this rolling wave-like earth-surface is formed of the outcropping edges of 

 the rock formations which are the special objects of study of the stratigraphical 

 geologist. If, therefore, the physiognomy of the face of our globe is any real index 

 of the character of the personality of the earth-crust beneath it, these collective 

 geographical features should be precisely those which answer to the collective 

 structural characters of the geological formations. 



In the earlier days of geology one of the first points recognised by our strati- 

 graphists was the fact that the formations were successive lithological sheets, whose 

 truncated outcropping edges formed the present surface of the land, and that these 

 sheets lay inclined at an angle one over the other, as William Smith quaintly 

 expressed it, like a tilted 'pile of slices of bread and butter.' But as discovery 

 progressed the explanation of the arrangement soon became evident. The 

 fomiations revealed themselves as a series of what had originally been deposited as 



