Seventeenth annual Meeting. 113 



bly struck with the fullness and accuracy of the description given by Prof. Lesquereux 

 in 1874, in his volume on the "Cretaceous Flora." Further, the more we look at the 

 work done by our late esteemed friend Professor Mudge, the more we are impressed with 

 the carefulness of his observations and the general justness of his conclusions. Hugh 

 Miller tells us that wherever he followed in the track of Sir Roderick Murchison, he 

 found all that geologist's work exact — the outline correct; only details to be filled in. 

 We believe it is so with Professor Mudge. What he did, was done, and can be taken as 

 a starting point for succeeding geologists. 



We have given some description, we have indicated some problems, but we cannot 

 now stay longer in the Dacotah, except to say that our description falls much below 

 the beauty and the wildness of this region; and that the solution of these problems will 

 add much, not only to our stock of knowledge, but also to the material wealth of this, 

 the Central State. 



NOTES OX THE GEOLOGY OF THE SPANISH PEAKS. 



BY J. SAVAGE. 



Like most volcanic mountains, the Spanish Peaks stand isolated and alone. They 

 are twins, and are known as the east or west peaks. When seen from a distance, their 

 outlines stand out sharp and pointed against the sky. The east peak is entirely volcanic 

 and destitute of mineral veins, and is the smaller of the two. 



The west peak has a volcanic nucleus, or cone, but has a thick outer covering or mantle 

 of the later sedimentary rocks. This outer blanket of soft sandstone has, by the intense 

 heat of the inner volcanic material, been metamorphosed into hard, silicious quartzite, 

 in which are found many metallic veins of the precious metals, besides abundance of 

 lead and traces of copper. 



Cotemporaneous with the elevation of the Spanish Peaks, and radiating from them 

 like the spokes from a gigantic wheel, are found a remarkable system of dykes. These 

 dykes are composed of varying volcanic material, and stretch off into the surrounding 

 country from ten to twenty miles in extent. They were thrust up through the soft, 

 friable sandstones of the Colorado lignite, in a soft, doughy state, leaving the impress of 

 the sandstone mould all along their entire length. Near the peaks, where the power of 

 erosion is much greater than elsewhere, the sandstone mould has been worn away, leav- 

 ing the harder and more silicious dykes still standing as plumb and straight as a masoned 

 wall, from 200 to 250 feet high; while further out in the surrounding plains, they have 

 weathered down into sharp, steep knobs and peaks, thus presenting a picturesque and 

 varied outline to the surrounding scenery. 



Fisher's Peak, near Trinidad, is a notable example of one of these dyke remains. 



The dykes, as they enter the west peak, are said by the miners to be converted into 

 true fissure veins, and from what I could learn during a short climb up the mountain, I 

 should judge their conjecture to be true. These mineral veins seem to penetrate the 

 entire mountain, and divide it up into regular sections. 



Near the base of the mountain, one dyke often cuts another in two, showing the one 

 thus cut to be the older, but still soft and plastic when the newer one divided it in twain. 



In approaching the peaks we find the strata gently dipping toward them, as though 

 the giving out from beneath of so much volcanic material as composes the peaks and the 

 dykes had left an empty space below, now occupied by the depressed strata. 



I know of no richer field for exploration by young geological students, in all the 

 Rocky Mountain region, than these Spanish Peaks, and the lessons learned from their 

 study may be summed up as follows: 



1st. All sharp-pointed mountain-peaks, whether seen from near or from far (and we 



