In our present state of ignorance respecting these mountains, it is impossible to give a solution of this phenomenon, though it may proceed from some distant volcano, which, like Stromboli, may be in a state of constant activity, but more irregularly. It is well known that the sounds of volcanoes are heard at very great distances, as at Guatimala, where the sound of the volcano of Cotopaxi was distinctly heard, though more than two hundred and twenty miles distant. Some indications of volcanoes had been seen by the American party, when ascending the river, about sixty miles below the mouth of the Little Missouri, where they passed several very high bluffs on the south side, one of which had been lately a burning volcano, as the pumice stones lay very thick around it, and emitted a strong sulphureous smell. Similar appearances are mentioned by Mackenzie, as taking place among the Rocky Mountains on their eastern side, in north latitude fifty-six and one hundred and twenty degrees west longitude. ‘Mr. Mackay,’ says he, ‘informed me, that in passing over the mountains, he observed several chasms in the earth that emitted heat and smoke, which diffused a strong sulphureous stench.’ From all these circumstances combined, it is natural to infer that the sound proceeds from some very distant and unknown volcano. On the west side of the Mississippi, and about midway between the Rocky Mountains and the Alleghanies, lies a broad range of mountains, called the Ozarks, six or seven hundred miles in length, about one hundred broad, and having an elevation varying from one to two thousand feet above the sea. This range of low mountains, which is penetrated by two branches of the Mississippi, the Arkansas and Red river, was nearly altogether unknown till within these few years. It is parallel with the range of the Alleghanies, making an angle of about forty degrees with the great range of the Andes. As far as the Ozarks have yet been explored, the granites and older primitive rocks are found at the lowest part, being surmounted by those of more recent formation. The reverse of this is observed in the Rocky Mountains. A similar range of broken and hilly country commences on the Ouisconsin river and extends north to Lake Superior. It is called the Wisconsin or Ouisconsin Hills. GENERAL REMARKS ON MOUNTAINS.