466 DESCRIPTIVE GEOLOGY. 



those forming these islands, and has, like them, been considered to belong to 

 the Lower Coal-Measure group, while the southern point is said to be formed 

 of a vesicular basalt. 



The Great Deseet. — To the west of Salt Lake and of the Cedar Mount- 

 ains, stretches a wide desert plain, formerly the bottom of the broadest 

 expanse of the ancient Lake Bonneville, and now known as the Great Desert. 

 Over an extent of some 40 miles in width by over 80 in length, its surface is 

 an almost absolute level, only broken by. a few isolated ridges, forming rocky 

 islands in the broad area. Its surface is covered by a deposit of the finest 

 silt and mud, the sediment of the ancient lake, and is almost absolutely 

 bare of vegetation. When dry, this mud forms a hard level floor, almost 

 like a pavem^ent, but a comparatively slight fall of rain renders it so soft as 

 to be practically impassable for horses or cattle. After a rain, the soluble 

 salts, with which the soil is impregnated, are left in a thin film of white 

 upon the surface, so that when looked upon from one of the bordering 

 ranges at a time when the sun is near the horizon, so that its rays are 

 reflected from the surface, the whole expanse seems to be covered by a 

 mantle of pure white snow. Along the borders of the desert, these Lower 

 Quaternary muds are covered by the more recent detritus of soil and gravel 

 brought down from the neighboring mountain-ridges, and on its eastern 

 side by the wind blown sands, which have accumulated to great depths 

 along the flanks of the hills which form the first barrier to the currents 

 brought along by the prevailing west winds that sweep its surface. The 

 difference of level between the mud plain and the present surface of Salt 

 Lake is so slight that its determination by barometrical measurements must 

 be received with some reserve. The elevation of the southwestern ex- 

 tremity, near Redding Spring, as determined by barometrical observations, 

 is 4,256 feet, or 56 feet above Salt Lake; but it is probable that a rise of 

 20 or 30 feet in the waters of this lake would submerge almost the entire 

 surface of the Lower Quaternary deposits. 



As might naturally be expected from the composition of the mineral 

 contents of the waters of Salt Lake, the salts which impregnate the soil of- 

 the Desert consist mostly of chloride of sodium; the surface incrustations 

 of this mineral are sometimes of sufficient thickness to form beds, notably 



