“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.’