Geology, Pfaff's Grundriss der Geologie, the author gives a brief account of the bearing of astronomical and mathematical investigations on the internal structure of the earth; and he very justly says that the results of observation compel us to regard the earth as for the most part fluid, in order to bring these results into harmony with calculation. Professor Pfaff attributes this conclusion to Hopkins, whereas it is precisely that which I had long since enunciated, and is entirely opposed to the views of Mr. Hopkins. More recently Sir William Thomson and Mr. Darwin have investigated the tidal action of an internal fluid nucleus upon its containing solid shell. They have both supposed the liquid to be totally incompressible, and the containing vessel to be elastic and therefore compressible. They have thus given the liquid a property which no liquid in existence possesses, and the solid a property which solids pos sess in a much less degree than liquids. Their hypothesis is thus totally inadmissible as a part of the problem of inquiry into the earth's structure. I at once admit that a thin elastic spheroidal envelope filled with incompressible liquid and subjected to the attraction of exterior bodies would present periodical deformations, owing to tidal action far surpassing the tides of the ocean. But I do not admit that such impossible substances can represent the materials of the earth. My hypothesis is that the liquid interior matter, instead of being incompressible, is, like all liquids we observe, relatively far more compressible than its solid envelope. A highly compressible liquid contained in a very much less-compressible shell would be a hypothesis more in harmony with physical observation. The tidal phenomena of a compressible fluid, it is easy to see, would be very different from those of an incompressible fluid. The work done by the action of certain disturbing bodies in the strata of compressible fluid would partly result in causing variations of density, instead of producing tidal waves of great Inagnitude. This has been already shown in the Mécanique Céleste by Laplace, in discussing the tides of the atmosphere. Theory shows that the atmospheric tides should be nearly insensible, notwithstanding the great depth of the atmospheric column, because the work done in the atmosphere is very dif ferent from what is performed in the less-compressible water of the ocean. Observation has fully verified this result.

3. It is admitted that the earth's density increases from its surface toward its center. If its interior is occupied by a compressible fluid, the law of density of this fluid would result from the compression of its own strata; just as the law of density of the atmosphere is produced by the pressure of the upper atmospheric layers upon those below. But instead of supposing the interior of the earth to be filled by a fluid thus con

forming to the observed properties of fluids, both Sir William Thomson and Mr. Darwin have applied their great powers as accomplished mathematicians to the tides of an incompressible and homogeneous spheroid, such as I admit to have no real existence whatsoever.

4. The labor bestowed on the problem investigated could scarcely be considered at all necessary or fruitful, except as affording an admirable illustration of the results flowing from the employment of hypotheses framed in direct contradiction to the fundamental conditions to which every truly philosophical hypothesis must conform. It is scarcely necessary to add, that the conclusions of Mr. Darwin, as well as those of Sir William Thomson, cannot be considered as having invalidated the carefully framed hypothesis that the earth consists of a solid crust physically similar to the rocks we are enabled to observe, and a contained spheroid of liquids and physically similar to the liquid rock poured out by volcanic openings.

5. It is with much satisfaction that I can trace a gradual growth of more correct physical views on the questions referred to in this paper. In Nature, vol. v, p. 288, à paper appeared in which I ventured to criticise Sir William Thomson's memoir on the Rigidity of the Earth, in the Philosophical Transactions. At the Meeting of the British Association in Glasgow, Sir William Thomson acknowledged the invalidity of many of his arguments, and requested his audience to draw their pens through paragraphs from 23 to 31 in his paper. These paragraphs contain statements and reasonings which I had already shown to be inconclusive in the paper which has just been quoted.

In Mr. Darwin's paper, recently communicated to the British Association, he admits that in discussing the precessional and tidal phenomena of a viscous liquid, the supposition of an elastic spheroid would lead to very different results-that is to say, results very different from those deduced by himself and Sir William Thomson regarding the earth's structure, and which the followers of the late Sir Charles Lyell have frequently assumed to be established. Thus the late Mr. Poulett Scrope appears to have referred to the bearing of the mathematical investigations alluded to, on what he calls "the sensational idea" of an internal incandescent fluid beneath the solid crust of the earth. He forgot that an idea may not be the less true because it is sensational. The idea of antipodes was at one time regarded as highly sensational. Those who witness a great earthquake or volcanic eruption are usually impressed with the sensational character of the phenomena.

6. A traveller who was in Portugal more than forty years since, met a woman over one hundred years of age, and asked

her if she recollected the great earthquake of Lisbon. She replied, that it was the event of all others in her long life which she ought to vividly recollect, on account of its impressive sensations. History also records the sensational character of the destruction of Pompeii. If Mr. Scrope's innuendo regarding the internal fluidity of the earth as "a sensational hypothesis" has any value, we should regard the events referred to as highly improbable; yet they have been as well authenticated as the most positive facts in science, and no person has ever expressed the smallest shadow of a doubt as to their occur

rence.

ART. LV.-Discoveries in Western Caves; by Rev. HORACE C. HOVEY, M.A.

THE following notes are selected from a large mass of descriptive material, collected by the writer during recent underground explorations in some of the States of the Mississippi Valley.

1. Silurian Caves.-Especial attention was paid to these caves in view of the "grave doubts" of a distinguished geologist "whether in a single case they extend much beyond the light of day."* His remark refers to the upper hundred feet of the Cincinnati group. An excellent opportunity for the study of caves in this Lower Silurian rock is afforded in bluffs about Madison, Indiana, which rise 400 feet from the thin strata characteristic of that formation, to the massive rocks of the Niagara limestone. Each stream, as it plunges down from the table-land above, washes out the lower layers, leaving the upper as an overhanging ledge. In time, the shallow grotto behind the cascade expands into a spacious amphitheater, 200 or 300 feet wide and nearly as many deep. The roof generally falls by its own weight when these dimensions are exceeded; and the result is finally a ravine with steep walls, encumbered below with large fragments of stone. An examination of the region for twenty miles north of Madison led to our discovering, not only sinks, natural wells, rock-houses and water-swept chasms, but also true caverns, whose roof is the solid limestone of the Upper Silurian, while the excavation itself is in the softer rocks of the Lower. Two miles west of Hanover, Indiana, is a stream that flows toward the Wabash from the very banks of the Ohio. It emerges from a tunnel which is easily threaded for half a mile, and continues uncovered for fifty feet; and then * Geological Survey of Kentucky (Shaler), vol. i, p. 4. AM. JOUR. SCI.-THIRD SERIES, VOL. XVI, No. 96.-Dec., 1878.

it again recedes by a second opening. We followed its course through roomy halls rich in stalactites to a waterfall fifteen feet high, where the exploration terminated. The entire distance traversed was, by estimate, one mile and a half-a greater length than that of Weyer's Cave. The credit of discovering this Silurian cavern belongs to Messrs. Monfort and Thomson; and as it is now for the first time described, it may be appropriately named the Hanover Cave.

2. Sub-Carboniferous Caves.-The procedure of the brook described above is reversed in the case of Lost River, which, after receiving tributaries and increasing in volume, flows into a cavernous opening and continues for miles along a subterranean channel, alternately rising to the surface and sinking again several times before it finally emerges a mile below Orangeville, Indiana. These "rises" as they are called, are generally marked by gulfs denoting the fall of superincumbent rocks; at one of them a small boat has been put upon the stream, it having been found to be navigable for a long distance under ground. Lost River flows amid bluffs of the Saint Louis group, carved by erosion into numerous ravines and sink-holes, and the latter so thoroughly underdrain the region as to cause a remarkable absence of springs, brooks and ponds.

These phenomena are instructive as to the production of the countless caves that honey-comb the Sub-carboniferous rocks of Kentucky and Southern Indiana. A compact and homogeneous limestone, varying from 25 feet to 440 feet in measured thickness, lies between the surface and the level of natural drainage, subject to the dissolving and eroding action of running water. The result, in time, is a succession of arches, galleries and avenues, presenting wonderful and grotesque combinations to the explorer when the stream that has caused them is withdrawn to some other channel. The slow trickling of limewater furnishes materials for the growth of stalactites that tend to gradually close up and obliterate these deserted halls. Should Lost River find another channel, the cave which would remain might equal in proportions any hitherto discovered. There are no doubt numerous unexplored and nameless caves that would richly reward those whose love of adventure should lead them to follow out their ramifications. Professor Shaler estimates that, in Kentucky, "there are at least 100,000 miles of open cavern beneath the surface of the Carboniferous limestone;" and my own observations lead to the conclusion that there are thousands of miles of such subterranean avenues beneath the same formation in Indiana. Yet the public should be cautious in yielding credence to cavestories. Articles appeared in Louisville papers less than a year ago, and were copied and believed in this country, and

even found their way into foreign periodicals, that purported to describe the "Grand Crystal Cave near Glasgow, Kentucky" giving thrilling particulars of a perilous voyage on its mysterious waters. We ascertained by inquiry on the spot that no such cave exists; and have learned by experience that cavestreams are generally very safe and placid bodies of water, by reason of the fact that they are not of navigable size until the level of adjacent streams is nearly reached.

3. Mammoth Cave is visited by more than 2,000 persons annually, and its noteworthy features have been repeatedly described. Tourists are usually content with either the Short or the Long route, both of which can be traversed in a single day. We, however, were favored with a special guide, and devoted many successive days to localities not often visited. After eighty miles of underground travel, our curiosity was satiated; and yet we had entered only 54 of the 225 avenues reported by Professor D. D. Owen as actually enumerated. The comparatively recent discovery of a pit-like passage called "the Corkscrew," is of importance, not only because it enables the visitors to cut off two miles between the Rotunda and River Hall by an abrupt descent of 150 feet; but also because it proves the theory that the cave crosses its own track, so that a change is required in the entire map. It is now believed that the cascade falling over the mouth and instantly sinking through the rocks is identical with that at the head of the River Styx, and is a feeder of that stream. It is also proved that these deep and navigable rivers, instead of being fed by Green River, flow into it. Chaff thrown upon the surface of Lake Lethe reappears after some time in the waters of what is known as the Upper Big spring; while that thrown upon Echo River comes out at the Lower spring. The fact that Green River is thus replenished explains the peculiarity of its never being frozen over even in the coldest winter. It may be added that as the waterlevel is known to be 312 feet below the crest of the hill covering the cave, the subterranean rivers must be at a little less than that number of feet beneath the surface, and must also be the lowest localities possible. Hence no dome in Mammoth Cave could exceed 312 feet in height without cutting through to the open air; by which test may be corrected the statements of those imaginative writers whose estimates are nearly double what they should be. The grandest of these vertical cavities, piercing from some sink-hole above through all the galleries down to the water level, is called, by way of eminence, the Mammoth Dome. Beyond it lies a stately hall, so like the ruins of Karnak and Luxor that we had permission to name it the Egyptian Temple. Here stand six columns of oolitic limestone encrusted with a stalagmitic coating but an inch or two