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."