There are some other volcanic bands which exhibit a similar parallelism with mountain chains; but, on the other hand, there are some volcanoes between which and the nearest mountain axes no such connection can be traced. [Sidenote: RELATION TO AREAS OF UPHEAVAL.] There is yet one other fact concerning the mode of distribution of volcanoes upon the surface of the globe, to which we must allude. It was first established by Mr. Darwin as one of the conclusions derived from the valuable series of observations made by him during the voyage of H.M.S. 'Beagle,' and relates to the position of active volcanoes with respect to the portions of the earth's crust which are undergoing upheaval or subsidence. From the relative position of the different kinds of coral-reefs, and the fact that reef-forming corals cannot live at a depth of more than twenty fathoms beneath the sea-level, or above tide-mark, we are led to the conclusion that certain areas of the earth's surface are undergoing slow elevation, while other parts are as gradually subsiding. This conclusion is confirmed by the occurrence of raised beaches, which are sometimes found at heights of hundreds, or even thousands, of feet above the sea-level, and of submerged forests, which are not unfrequently found beneath the waters of the ocean. By a study of the evidences presented by coral-reefs, raised beaches, submerged forests, and other phenomena of a similar kind, it can be shown that certain wide areas of the land and of the ocean-floor are at the present time in a state of subsidence, while other equally large areas are being upheaved. And the observations of the geologist prove that similar upward and downward movements of portions of the earth's crust have been going on through all geological times. Now, as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we trace upon a map the areas of the earth's surface which are undergoing upheaval and subsidence respectively, we shall find that nearly all the active volcanoes of the globe are situated upon rising areas, and that volcanic phenomena are conspicuously absent from those parts of the earth's crust which can be proved at the present day to be undergoing depression. CHAPTER IX. VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY.