LETTER. • 3 



ions of the party; the main portion of the work being done from topo- 

 graphical stations situated upon the most commanding points within the 

 region, together making a system of secondary triangles within the pri- 

 mary system. From these commanding points, of which seventy, averag- 

 ing about eight miles apart, were occupied during the season, both drain- 

 age-sketches expressed in contours, and perspective profile-sketches, 

 were made by the topographers, and angles, both horizontal and vertical, 

 taken to all the intersections of streams, their principal crooks and 

 bends, all points, spurs, saddles, and sudden changes of slopes; whicb, 

 with similar angles taken from adjacent stations upon the same points, 

 served to fix their positions and elevations; and with the thousands of 

 points thus fixed during the season with an exceedingly close approxi- 

 mation to absolute truth, the map is filled in from sketches made by 

 the topographer. 



The geologist, meanwhile, has made his own detailed and special 

 studies and sections along the route traveled, or on special trips for this 

 purpose ; and this detail he generalizes from the higher stations made 

 by the topographer, obtaining extensive views from them, from which 

 he can trace his formations across the country; and with the locations 

 and directions furnished by the topographer more accurately and readily 

 than he can obtain them himself, he can secure data by which he can 

 readily color a general geological map upon his return. Indeed, next 

 to entering the field with a finished topographical map of the region to 

 be examined, a thing as yet impossible in our West, the union of topog- 

 raphy and geology in one and the same party best furnishes the data 

 for a realization of the full value of the otherwise more or less discon- 

 nected observations of the geologist ; and equal benefits accrue to the 

 topographer, for mannerism and inexpressiv^e effects are inevitable 

 results when a topographer sees but the surface of a country and under- 

 stands not its anatomy. Association of topographer and geologist thus 

 leads to benefits to each, and is certainly a great advantage to the sys- 

 tem of field-working now followed by the survey. 



The first chapter of his report deals with the main drainage-systems 

 and principal topographical features of the whole district, which mate- 

 rial is greatly supplemented by the report Mr. Ladd has prepared upon 

 the means of communication, elevations, distribution of timber, grass- 

 lands, and population. 



The Sedimentary rocks underlying the great plains are all thrown up 

 along the mountain-base, with their edges exposed. The formations 

 thus exposed form the subject of Chapter II, where they are treated in 

 order from the lowest, the Triassic, resting directly on the Archaean 

 rocks, through the Jurassic, Cretaceous, and Lignitic (Upper Cretaceous 

 or Eocene,) with a few facts about the more recent gravels and lavas. 



The detail- characters of the lower beds are exhibited (Plate I) in detail- 

 sections made at six points along the front of the mountains, while their 

 structural features — the more simple fold of the south, the more com- 



