Since the largest and most important caves occur in limestone, a little special attention to the cause of their occurrence there may serve to show that although speleology has only recently received its name and been elevated to the rank of a separate and independent science, it is one of the earth's ancient institutions.
Our geologists, who have unearthed many secrets not dreamed of even in Humboldt's "good phylosopy," have settled the question of how the different kinds of caves were formed, according to the character of rocks they are in, or their location and depth, and the natural agencies to whose action they show signs of having been subjected.
Dr. H. C. Hovey, in his "Celebrated American Caverns," says: "In visiting caves of large extent, one is at first inclined to regard the long halls, huge rifts, deep pits and lofty domes, as evidences of great convulsions of nature, whereby the earth has been violently rent asunder. But, while mechanical forces have had their share in the work, as has been shown, the main agent in every case has been the comparatively gentle, invisible gas known as carbonic acid. This is generated by the decay of animal and vegetable substances, and is to a considerable degree soluble in water. Under ordinary circumstances one measure of water will absorb one measure of carbonic acid; and the eye will detect no difference in its appearance. Under pressure the power of absorption is rapidly increased, until the water thus surcharged has an acid taste, and effervesces on flowing from the earth, as in Saratoga water.
"Rain-water, falling amid leaves and grass, and sinking into the soil, absorbs large quantities of carbonic acid. On reaching the underlying limestone, the latter is instantly attacked by the acidulated water in which it is dissolved and carried away.
"It is agreed among geologists, amazing as the statement may seem, that the immense caverns of Virginia, Kentucky and Indiana, including Mammoth Cave itself (the largest of all), were eaten out of the solid mass of limestone by the slow, patient, but irresistible action of acidulated water."
Professor N. S. Shaler says: "The existence of deep caverns is a sign that the region has long been above the sea."
Through the kindness of Professor C. J. Norwood, Chief Inspector and Curator of the Geological Department of Kentucky, it is possible to quote the first official report made on the caves of that state and published in 1856, in Volume I., Kentucky Geological Survey Reports. Dr. Norwood says: "Referring to the 'Subcarboniferous Limestone' (now known as the St. Louis group of the Mississippian series), Dr. Owen says: 'The southern belt of this formation is wonderfully cavernous, especially in its upper beds, which being more argillaceous, and impregnated with earths and alkalies, are disposed to produce salts, which oozing through the pores of the stone effloresce on its surface, and thus tend to disintegrate and scale off, independent of the solvent effects of the carbonated water. Beneath overhanging ledges of limestone, quantities of fine earthy rubbish can be seen, weathered off from such causes. In these I have detected sulphate of lime, sulphate of magnesia, nitrate of lime, and occasionally sulphate of soda. The tendency which some calcareous rocks have to produce nitrate of lime is, probably, one of the greatest causes of disintegration.'"
"Most extensive subterranean areas thus have been excavated or undermined in Edmonson, Hart, Grayson, Butler, Logan, Todd, Christian and Trig. In the vicinity of Green River, in the first of these counties, the known avenues of the Mammoth Cave amount to two hundred and twenty-three, the united length of the whole being estimated, by those best acquainted with the Cave, at one hundred and fifty miles; say that the average width and height of these passages amount to seven yards each way, which is perhaps near the truth; this would give upwards of twelve million cubic yards of cavernous space which has been excavated through the agency of calcareous waters and atmospheric vicissitudes."
Page 169: "On the south side of Green River the platform of limestone forming the descent into Mammoth Cave is two hundred and thirty-two feet above Green River."
"The entrance to the cave, being thirty-eight feet lower than this bed of limestone, is one hundred and ninety-four feet above Green River. In the above two hundred and thirty-two feet there are several heavy masses of sandstone, viz.: at one hundred and twenty-five, one hundred and forty-five, one hundred and fifty, one hundred and sixty and two hundred and fifteen feet, but it is probable that most of these have tumbled from higher positions in the hill, as no alterations of sandstone have been observed at these levels in the cave. From an elevation of from two hundred and forty to two hundred and fifty feet, the prevalent rock is sandstone without pebbles, which can be seen extending up to three hundred and twelve feet to the foundation of the Cave Hotel. The united thickness of the limestone beds on this part of Green River, is about two hundred and thirty feet, capped with eighty feet of sandstone. About midway of the section on this part of Green River, are limestones of an obscure oolitic structure, but no true oolite was observed. Many of these limestones are of such composition as to be acted on freely by the elements of the atmosphere, which, in the form of nitric acid, combine with the earthy and alkaline bases of calcareous rock, and give rise to the formation of nitrates with the liberation of carbonic acid; hence the disintegrated rubbish of the caves yields nitrate of potash after being treated with the ley of ashes and subsequent evaporation of the saline lixivium. The wonderfully cavernous character of the subcarboniferous limestones of the Green River valley, and, indeed, of these particular members of the subcarboniferous group throughout a great part of its range in Kentucky and Indiana, is due in a great measure to this cause, together with the solvent and eroding effects of water charged with carbonic acid. The 'rock-houses' frequently encountered both in this formation and in the limestones of Silurian date, are produced by similar causes; the more easily disintegrated beds gradually crumbling away, while the more durable remain in overhanging ledges. By the oxidation of other elements, sulphates of oxide of iron and alkalies result, which, by double decomposition, with carbonate of lime, give rise to the formation of gypsums which appears in the form of rosettes, festoons and various other imitative forms on the walls and ceilings of the caves. Crystallizations of sulphate of soda and sulphate of magnesia are not uncommon, both in some of the caves and in sheltered situations under shelving rocks."