MERRIAM: TKIASSIC ICHTHYOSAURIA. 



15 



probably in Nevada also, but the conditions favoring the preservation of fos- 

 sils are not similar in corresponding horizons of the two regions. For this 

 reason it has chanced that ichthyosaurs are known as yet only from the middle 

 division of the Trias in Nevada and from the upper division in California. In 

 the fossiliferous marine Lower Triassic limestones of California and Idaho no 

 remains of reptiles have yet been obtained, though marine fishes are common in 

 the Lower Triassic of Idaho. 



The geological correlation of the marine Triassic of the Pacific Coast region 

 of America has been made the subject of careful investigation by Professor 



Fig. '2. Sketch map of northern California and northwestern Nevada, giving occurrence of Triassic 

 formations in which ichthyosaurians have been obtained; VT, Upper Triassic; MT, Middle 

 Triassic. 



James Perrin Smith, from whose work on the stratigraphy of the West Amer- 

 ican marine Trias :i the table on the following page has been extracted. 



The Upper Triassic beds containing ichthyosaurian remains in California 

 are largely exposed in Shasta County between the west side of Pit river and 

 Squaw creek. 4 Large exposures form the imposing gray cliffs and peaks along 

 the top of the prominent series of points known as Brock mountain (pi. 1, fig. 

 12). The outcrops also cross Pit river to the south, and extend to the north in 

 rather narrow exposures beyond the headwaters of the North Pork of Squaw 

 creek. The formation consists principally of the phase known as the Hosselkus 

 Limestone. It is rather sharply divided lithologically into an upper and a 



" Smith, Jas. Perrin, Festschrift zum siebzigsten Geburtstage von Adolf v. Koenen, 1907. 

 * Diller, J. S., U. S. Geol. Surv., Bedding folio. 



