OCR Text |
Show 202 GEOLOGY OF THE HIGH PLATEAUS. The different stages of the decay are readily discerned, and it is easy to see that the various basaltic eruptions, though they may, in a certain geological sense, be considered as belonging to one epoch, and that a very recent one, have occurred at intervals which, measured by a historical standard of time, have been very long. The lithological characters also vary to some extent; the more ancient floods being less heavily charged with magnitite, and on the whole less basic and a little lighter-colored, also less finely textured, than the most recent ones, and of a little lower specific gravity. Finally, the largest basalt field of all and, with the exception of that one nearest to Panquitch Lake the most recent, is found near the southwest margin of the plateau, covering about 25 square miles, with a considerable number of cones, from which a large number of eruptions have issued. This field I have had no opportunity to examine in detail, and it is not easily accessible on account of the exceedingly rough character of its surface. Much of it is clothed with dense forests of spruce, which alone render it almost impenetrable, and prevent the observer from obtaining a satisfactory view of it, Its mean altitude is more than 10,000 feet. The basaltic eruptions of the Markagunt are a portion of a belt of such eruptions, which extends along the course of the Hurricane fault and the country adjacent to it far southward across the Colorado River into Arizona, Eruptive rocks older than basalt within this belt are very few and of small magnitude. The volume and number of basaltic eruptions increase as we proceed southward, and reach a great development near the Grand Cation, where more than a thousand square miles are covered with it and more than a hundred cones are still standing. South of the Colorado many large basalt fields are known to exist, but they have not been thoroughly studied. Throughout the Hurricane belt they occur in patches, often small, but frequently extensive. It is a notable fact that by far the greater portion of them occur upon the uplifted side of this great displacement; indeed those upon the thrown side are comparatively trivial. This fact seems to be generally true throughout the District of the High Plateaus and also throughout the country to the south of it. It is, moreover, so strongly emphasized, that it suggests the possibility of a correlation between these basaltic eruptions and the greater upward displacements. |