202 GEOLOGY OP 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 tex- 

 tured, than the most recent ones, and of a little lower specific gravity. 



Finall)', the largest basalt field of all and, with the exception of that 

 one nearest to Panquitch Lake the most recent, is found near the south- 

 west margin of the plateau, covering about 25 square miles, with a con- 

 siderable 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 Hunucane 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 Cafion, 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 thor- 

 oughly 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 displace- 

 ment; indeed those upon the thrown side are comparatively trivial. This 

 fact seems to be generally true throughout the District of the High Pla- 

 teaus and also throughout the country to the south of it. It is, moreover, 

 so strongly empliasized, that it suggests the possibility of a correlation 

 between these basaltic eruptions and the greater upward displacements. 



