the Muddy Mountains, Nevada, etc. 53 



ment to another is often abrupt horizontally as well as 

 vertically. Limestone and magnesian beds make up 

 about a third of the thickness, and form the most con- 

 spicuous part of the formation. Much of the limestone 

 is practically pure calcium carbonate, lies in heavy, regu- 

 lar beds, and has a delicate pink color on fresh surfaces. 

 Oolitic or concretionary structure is common. No fossils 

 were found, and it is probable that the limestone formed 

 as a chemical precipitate in basins of brackish water. In 

 many parts of the area the limestone horizon is occupied 

 by 100 to 300 feet of a snow white deposit which is almost 

 pure magnesium carbonate ; and in other sections the two 

 types of deposit interfinger. Thin beds of volcanic ash 

 occur at short intervals in the limestone and ^^magnesite" 

 and are also found at higher horizons. 



For this series of beds the name Horse Spring forma- 

 tion is proposed, because of the prominent exposures near 

 Horse Spring, in the Virgin Mountains. No fossils were 

 found in these deposits or in the Overton fanglomerate, 

 and their exact age is therefore undetermined. In lith- 

 ology, in degree of deformation, and in general regional 

 relationships the Horse Spring deposits compare well 

 with the Siebert and Esmeralda formations of southwest- 

 ern Nevada. Both of the latter formations are of Upper 

 Miocene age, and the Overton and Horse Spring sedi- 

 ments will be referred tentatively to the same epoch. 



All of the large intermontane valleys contain thick de- 

 posits of silt, clay, and sand which have suffered much 

 less deformation than the next older formations. Wher- 

 ever contacts were observed the intermontane sediments, 

 lying horizontally or only moderately folded, rest on the 

 bevelled edges of steeply tilted Horse Spring and Overton 

 beds. For the most part the silt is in massive layers 6 

 inches to 2 feet thick, separated by lenticular layers of 

 sand imperfectly cemented. Clay free from grit occurs 

 in very minor quantity. Adjacent to the mountains the 

 base of the sediments consists of coarse, unassorted 

 gravel ; but at a higher horizon the finer materials are in 

 direct contact with solid rock ridges and have thin lam- 

 ination, indicating that at least a part of the deposition 

 occurred in lakes deeper and more permanent than playa 

 lakes. In the Virgin Valle}^, beds of rock salt 50 to more 

 than 100 feet thick occur in the lower 400 feet of the sedi- 

 ments, and gypsum layers are common through a much 



