XXII LETTEE TO THE SECRETAEY. 



and their heights are known ; we have also a profile stetch of the spur. Given these 

 data, and what is easier than to distribute the curves, with a considerable approach 

 to accuracy, between those points actually determined ? 



This method of representiug orography is a strictly natural one. Supposing the 

 light to be vertical, grade curves must necessarily produce the same lights and shades 

 as in nature. All the details of the topography, down to forms not above 200 feet in 

 height, may be expressed. 



For maps for the representation of geological outcrops and formations, these grade 

 curves are invaluable. They enable the geologist to draw accurately the outcrops 

 not only of horizontal, but of inclined strata, over vast areas, from a few isolated 

 observations. In many cases, by thus expressing the orography of a range, the key 

 to its geological structure is supplied. 



Extended notices of these maps, both topographical and geological, were published 

 in the American Naturalist for January and March, 1880 ; and a small edition of them 

 was distributed to working geologists and scientific societies in this country and in 

 Europe. 



These maps, as well as those of the Colorado Atlas, were engraved by Mr. Julius 

 Bien, of New York City, and they have justly been regarded as among the finest speci- 

 mens of cartography ever published in this or any other country. 



For the main geological features of the Park the reader is referred to 

 the admirable report of W. H. Holmes in this volume. As he has stated 

 in his preface, it is far from being exhaustive and many imjiortant prob- 

 lems are left unsolved, yet it will certainly be regarded as a real addi- 

 tion to our knowledge of this most interesting area. To make an ex- 

 haustive study of this geological district must be the work of years 

 instead of a few weeks. The geological map accompanying this vol- 

 ume will be found to be a great improvement upon the one published 

 in 1872. Considering the shortness of the time occupied in the field 

 work, the detailed accuracy of the various formations is remarkable. 

 This could have been done by Mr. Holmes only through his great skill 

 in sketching rapidly the physical features. Some additional matter on 

 the geology will be found in the annual reports of the Survey for 1871 

 and 1872. 



Although most of the geological formations known to occur in the 

 Northwest are represented in the Park, it will be observed that rocks of 

 volcanic origin cover far the greater portion. Ehyolite ijredomiuates 

 over all the other varieties. The areas colored as volcanic tertiaries 

 are really conglomerates or breccias of volcanic origin, and comiDosed 

 mostly of volcanic rocks. In determining the variety and character of 

 these rocks, of which Mr. Holmes made a large collection, he received 

 the able assistance of Capt. 0. B. Button, whose report is appended, 

 The volcanic conglomerates form a very conspicuous feature of the Park, 

 reaching a thickness in jjlaces of two to four thousand feet. Two of 

 the most important ranges of mountains in this region, the Yellowstone 

 and Washburn ranges, are almost entirely composed of these conglom- 

 erates. It will be readily seen that they open up an almost limitless 

 field of study. 



At the present time the older sedimentary groups, from the Cretaceous 

 to the Silurian, occupy comi)aratively small areas; formerly they prob- 



