The precise manner in which the plastic material inside of the terrestrial shell gets access to the surface, is not entirely clear. Nevertheless, it is possible to get some light on the matter. It is now well known that in many places there are deep cracks, or "faults," in the earth's crust. Some of them in the remote past have been wide and deep enough to admit molten material from below. The Palisades of the Hudson are believed to have been formed by such an intrusion, the adjacent rock on the eastern face having since been worn away by the weather or other agents. It has been observed that many volcanoes are distributed along similar faults. The existence of a chain of volcanic islands in the West Indies suggests the probability that it follows a crack of great antiquity, though the issue of lava and ashes for several centuries may have been limited to a few isolated points. Just how these vents have been reopened is one of the most difficult questions still left for investigation. Given a line of weakness in the rocks, though, and a susceptibility to fresh fracture is afforded. Professor McGee suggests that the overloading of the ocean bed by silt from the Mississippi river or other sources may have been the immediately exciting cause of the recent outbreaks. Other geologists have found a similar explanation acceptable in the case of eruptions elsewhere. The theory has much to commend it to favor. The Martinique disaster already has drawn from geologists and volcanists many expressions of opinion, and explanations of volcanic phenomena which set forth in detail the causes and effects of volcanic eruptions, in particular, and seismic convulsions, in general. Dr. A. R. Crook, a professor in Northwestern University, has made a special study of volcanoes. He has made an ascent of the two highest in the world, and has climbed many others for purposes of study. He is an authority upon volcanography. "There are two great circles of volcanoes about the earth," said Professor Crook. "One girdles the earth north and south, extending through Tierra del Fuego (called 'land of fire' because of its volcanoes), Mexico, the Aleutian islands and down through Australia; the other east and west through Hawaii, Mexico, West Indies, Italy (including Mount Vesuvius) and Asia Minor. "These two circles intersect at two points. One of these is the West Indies, which include Martinique, the scene of this terrible disaster; the other is in the islands of Java, Borneo and Sumatra. On the latter islands there are extinct volcanoes. On the former is the terrible Pelee. It is just at these points of intersection of the two volcanic rings that we expect unusual volcanic activity, and it is there that we find it.