S. F. Emmons — Little Cottonwood Granite. 139 



Aet. XIII. — The Little Cottonwood Granite Body of the 

 Wasatch Mountains ; by S. F. Emmons. 



The Wasatch Kange, from a geological standpoint, is the 

 most important of the many more or less parallel mountain 

 uplifts that go to make up the Cordilleran Mountain System 

 in its widest part — that is, between the latitudes of 39° and 

 41° I^. It has, from the earliest geological time of which we 

 have any record, formed the dividing line between the geology 

 of the west or Pacific slope and of the eastern Rocky Moun- 

 tains, there being an essential difference on either side of this 

 line, not only in the lithological constitution of the respective 

 geological formations but in their faunal characteristics. 



From a Tectonic point of view it is equally important, pre- 

 senting as it does examples of most of the varied phenomena 

 involved in mountain building, the latest phase having been a 

 great meridional fault which has cleft it in twain, so that its 

 western half is now buried beneath the floor of the Great Salt 

 Lake Yalley ; it is thus well worthy of the characterization 

 given it by the elder Dana, as "the grandest exhibition of 

 facts pertaining to an individual case of mountain building in 

 geological literature."* 



It was in the summer of 1869 that the geologists of the 4:0th 

 Parallel undertook the geological examination of this range. 

 The two previous seasons had been devoted to geological map- 

 pingf of the Desert Ranges of ]S"evada and western Utah. In 

 these isolated ridges, standing like islands in a great ocean of 

 recently deposited sediments, the rocks were found to be mainly 

 eruptives and members of a great Paleozoic series whose 

 extent, position, and faunal characteristics in the vast region 

 west of the Mississippi Yalley were as yet completely unknown, 

 and the facts gathered so far had afforded no clue whatever as 

 to the extent or order of superposition of the different mem- 

 bers of this portion of the sedimentary column. 



As our weary march across the desert in the season of 1869 

 had finailly, in the month of November, led us along the 

 western foot of this magnificent range through the smiling 



*This Journal (3), xlv, 181. 



fit is perhaps questionable whether the word "mapping" is the correct 

 term in this place, for at the time no maps of the region were extant and 

 topographer and geologist did their field work side by side. It was not until 

 1875 that the paleontologists had furnished their final determinations of the 

 age of the various groups of fossils collected and the topographers had com- 

 pleted their maps so that the geological outlines might be laid down upon 

 them. 



Am. Jour. Sci.— Fourth Series, Vol. XYI, No. 92.— August, 1903. 

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