The speaker, armed with this formidable verbal equipment of attack upon his audience, had walked to the front of the platform, and, harboring some unusual confidence in his powers, had deserted his manuscript. _Isostacy_, he had realized, possessed probably unqualified novelty, and by way of assurance, lest its terrors might empty the hall, he assumed a colloquial relation to his dazed hearers, and offered an explanation of this unexpected mystery. “Isostacy,” he resumed, “is simply this: Equilibrium. It is the maintenance of average level--as if one part of the earth’s surface was pushed up, above a mean level, then the requirements of Isostacy would depress another part, below it. We can also call it the adjustment of a changing load, as if through depression, from the dumping upon the floor of the ocean of a great amount of sediment, derived from the land surface of the earth, neighboring areas of the land of the oceanic floors were raised. Two contiguous regions _might_--and,” the lecturer turned directly toward the President, who in his own earnestness of attention had elbowed himself round into a direct line with Mr. Binn, “in the case of the West Indian continent and the Isthmus of Panama, _have_ maintained between them, an up and down reciprocity of movement, as, when one was up, the other was down, and vice versa.” Mr. Binn looked introspectively at the walls and ceilings of the room, as if engaged in a mental rehearsal and review of his staggering statement, and returned to his desk and manuscript, satisfied that he had thrown the assembly into an uneasy apprehension of danger. He again began his reading: “It is true, if I understand Mr. Spencer correctly, that the Atlantic ocean was cut off by the elevation of the Columbian Continent from even the interior basins of the Caribbean Sea and the Gulf of Mexico, at least in early pliocene times; that these depressions were then broad plains receiving in part the drainage of the Antillean highlands; this again emptying into the Pacific ocean. But this is not a proven theory, and it involves an extravagant readjustment of the physical features of a region that to my mind more expressively can be considered immemorially permanent, in their general aspects, at least. I reiterate the reciprocity of movement between the Antillean Continent and the Isthmus of Panama. The cause I have suggested may be untenable--but there seems strong geological proof of some such alternating relation between the west and east sides of this inter-related region, the Great Antilles on one side, the Isthmus of Central America on the other.