The Evacuation of England: The Twist in the Gulf Stream

by L. P. Gratacap

“Now of all these types of structures the last obviously best meets the requirements of a type which will prove the least susceptible to dislocation. I think that can be apprehended almost without explanation. A moment’s reflexion will make it conspicuous.

“The bricks tilted up in inclined tiers or beds, upon disturbance, if the cohesion between them is seriously impaired, tend to fall away from each other, and gravity increases the effects of the initial displacement. If the bricks lie flat they do not fall apart, upon the cessation of any push or upheaval, but remain disordered, falling back into some quasi-position of rest. If the bricks are inverted and form in section a series of lines converging to the base of the wall, their disarrangement is largely rectified by their own gravity, bringing them back into their first positions.

“In Geology strata overlying each other, in succession, as the bricks do when on their flat faces are called conformable, if they succeed, one over the other, with the edges or summits of the lower, abutting against the horizontal surfaces of the next, as do the bricks when they are placed in flat and vertical positions, in alternating strips, that is unconformability.

“If the strata are usually horizontal like the evenly piled series of bricks, they are called undisturbed; if inclined against each other, they are inclined, and they may make monoclinals, having one slope, or anti-clinals when they lean up against each other like the opposite sides of a peaked roof; or synclinal when inclined towards each other in an inverted position like the same roof overturned, with its ridge pole on the ground, and its inclined sides lifted into the air, or like the bricks in the last pattern of structure described.

“When we carry these similes into nature, we have all kinds of rocks, and we have them in mountains, in planes, and all the familiar configuration of the earth’s surface.

“Now we find that those portions of the earth immediately beneath our feet, extending for a mile or so into the surface of the earth, are variously made up of layers, strata, beds, formations, lying on one another, and conformable or unconformable, undisturbed or thrown into anticlinal or synclinal folds; that the material in its general mineral character, is limestone, marls, or sands and sandstone, slates, clays, metamorphic rocks like gneiss and quartzite, etc., and associated with them are granites which may have been melted lava-like rock before it cooled and crystallized, while there is plentiful evidence of abundant outflows of igneous, melted or viscid rocks; evidences of lines of eruption, of foci, or craters of eruption. Thus, as in the brick structure, where unrelated and later material has been introduced in fissures, gaps, openings, holes, etc., of the walls, we have some of the architecture of the earth, an original bedded structure invaded by very contrasted substances, and which give to that architecture, as in the brick wall of our homely illustration, lack of homogeneity, and lack of strength.

“In the West Indies and on the Isthmus of Panama we have the states of instability which we have signalized, viz., secondary deposits of a somewhat loose and unconsolidated material, and wanting in the deeply bedded crystalline rocks which in New England, in the Adirondacks, and the Piedmont or higher regions abutting on the coastal plain in the northern United States, furnish a solid, and probably fundamentally deep seated pediment of resistance to shock. Again in the West Indies and in the Isthmus, we have the beds unconformable over each other, which you will recall in our symbol of the brick wall, was a feature of weakness; also these unconformable beds are inclined in anti-clinals, a further aspect of structural insolvency; and further these beds have been widely, pervasively, in places, infiltrated and ruptured by subsequent introductions of volcanic substance, ashes, lavas and intrusive magmas. Thus the geological aggregates present the previously illustrated condition of fragility, and the absence of the so-called tectonic elements of rigidity. But still one step more in our disheartening study of this equatorial problem.

“I, a few moments past, called your attention to the fact that ‘if this least massive and most vulnerable type of structure has been subjected to repeated and considerable strains of elevation and depression, and strains recurrent at short intervals, then, without inspection, we know that its interior has been much shattered, and that it has undergone a progressive dilapidation.’

“Precisely such catastrophes are discovered in the history of the geological region now before us. The islands of the West Indies have been subjected to great changes of elevation. They have risen and fallen during the last geological age—the Tertiary—perhaps four times. In their rise they have gathered to themselves marginal extensions of land, now hidden beneath the ocean at comparatively slight depths, while they have at the same time doubtless become blended and unified into a great Antillean continent. This continent was dominated by volcanic protuberances whose growth upward, over accumulations of ashes, has been again symptomatic of undermining operations threatening later subsidence and submergence.

“In our day we have been called on to deplore the ravages caused by the eruptions of Mt. Pelee and La Soufriere, on the Islands of Martinique and St. Vincent, and it is natural to insist that regions which have a precarious autonomy, in which such volcanoes can exist, must be regarded with diffidence, as permanent geographical areas.