1916] Martin: Pliocene of Middle and Northern California 249 



very greatly unless they are eliminated. A small number of species 

 from one formation will change the factors considerably. It is 

 necessary therefore to have a large number of species from all of 

 the formations which are to be considered. Geographical separation 

 also affects the results very noticeably. Formations that are widely 

 separated will have a smaller number of species in common than 

 formations that are not so situated. 



With our data tabulated as in the following tables, and with 

 the possibility of errors in mind, we can readily construct a 

 correlation table which will show the relations of each formation as 

 worked out by the percentage method. Turning to table 3 we find 

 in the first column, reading from top to bottom, the percentage of 

 living forms. The formation having the greater per cent of living 

 species will, of course, be the most recent. Those having the same 

 percentage of living forms will be approximately contemporaneous 

 in deposition. Considering then the percentage of living species we 

 find that the upper Merced is very recent, certainly Pleistocene. 

 The Merced of Bolinas Bay, of Alio Nuevo Bay, of Seven Mile Beach, 

 of Pillar Point, and of Sargent Oil Field are all of the same age and 

 are very closely related to the upper Wildcat and to the Purisima. 

 The Etchegoin of the Sargent Oil Field, the lower Wildcat, and the 

 so called Miocene of Bear River are older. 



Taking now the factors representing the percentage of species 

 common to the formation among themselves, we find that a similar 

 relation will work out. Comparing the formations to the upper 

 Wildcat we find that the largest factors correspond to the Merced 

 of Seven Mile Beach and to the Merced of Sargent Oil Field. If we 

 compare the formations to the Etchegoin of the Sargent Oil Field 

 we find that the Purisima is more closely related to it than the Mer- 

 ced or the Wildcat Series. By comparing each of the formations in 

 this manner a relation would be worked out which would agree very 

 closely with that shown in the column of the percentages of living 

 species. 



In correlating by means of the most characteristic species we 

 find that the Etchegoin of the Sargent Oil Field contains a number 

 of the short-lived forms which occur abundantly in the Etchegoin of 

 the Coalinga district. We are justified, therefore, in correlating 

 these formations and assigning them to the same horizon. A few of 

 the short-range species of the Etchegoin fauna occur commonly in 

 the Purisima, and it seems probable that a portion of the Purisima 



