6 GEOLOGICAL SUEVEY OF THE TERRITORIES. 



This range reacbes out from near the headwaters of the Gunnison 

 E-iver, forty miles to the northwest, and, though not generally as high as 

 the other ranges of Northern Colorado, is by far the most rugged of them 

 all. The reasons for re-examining this range, when it had been sur- 

 veyed in the regular progress of the work, were two : first, their rug- 

 gedness and inaccessibility had made the dilficuity of working them last 

 year so great, that they were not survej^ed in a style quite up to the 

 standard of the remainder of the work ; and, secondly, that their geo- 

 logical importance made it a matter of particular scientific interest that 

 they should be carefully studied and mapped. 



The geologist and topographer worked side by side through them, 

 making forty-two high mountain-stations ; Mr. Holmes sketching the 

 different portions of the whole mass from as many points as possible. 

 They contain about eight hundred square miles, and will be mapped on 

 a scale of one mile to the inch. 



Marching from here by way of the Twin Lakes and South Park to 

 Caijon City, we carried on a running survey along our route, and from 

 the latter place commenced work, on the last part of the summer's plan, 

 the mapping of the sedimentary border-line and that of the coal from here 

 to the Wyoming line. This work, carried on without interruption, was 

 finished by Mr. Chittenden, Mr. Holmes, and myself on the 20th of 

 October ; it having required seventy- four topographical stations. This 

 survey was of a great deal of practical as well as scientific importance, and 

 of immediate need, since, in the coal-series, we were enabled to lay down 

 pretty closely that broken winding line more than two hundred miles in 

 length inside of which no coal might be found. The labor involved in 

 carefully laying down this line cannot be realized until one notices the 

 admost numberless prospect-holes that have been sunk into the worth- 

 less black shales which, all along the base of the mountains, lie inside 

 the coal-series and tempt the settlers into profitless investments and 

 unrequited diggings after coal. 



In carrying on this last survey, the Land-Office work has been of 

 great assistance, and also the careful studies of Captain Berthoud of 

 the coal lying to the west and north of Denver. 



The maps produced from this special survey and included in this 

 report are as iollows : — 



1. A map of the eastern base of the mountains from below the Arkansas 

 Eiver to the northern line of the Territory, on a scale of two miles to 

 one inch. On this map.are plotted the line of coal-outcrop, the junction 

 of the sedimentary and metamorphic rocks, and the inner limits of the 

 Cretaceous. 



2. A map of the Elk Mountains on a scale of one mile to an inch, plot- 

 ted with 200-foot contours.* 



3. A map of the Upper Arlcansas Valley, showing the heavy morainal 

 deposits in the vicinity of the Twin Lakes, on a scale of one mile to an 

 inch. 



'These maps are all reduced one-balf for publication. 



