440 THE JOURNAL OF GEOLOGY. 
to have commanded but little attention. They were recently 
described more in detail by Professor Lawson, by whom they 
were called the Merced series.‘ They have been called Pliocene 
by all who have worked on them. From Half Moon Bay south- 
ward at a number of places occur very fossiliferous beds which 
have often been referred to, but of which very little has been 
written. The fossils obtained from these beds have been referred 
by some to the Miocene, by others to the Pliocene. The same | 
beds have been referred to the Pliocene where little disturbed, 
and to the Miocene where much disturbed. 
The field-work of the writer has shown: 
1. That, though minor oscillations have occurred, there was 
continuous sedimentation from the beginning of the Monterey 
period to the end of the Merced. 
2. That the two series are similar in structure, that structure 
having been determined by the movement which took place at 
the end of the Merced period. 
3. That the fossiliferous beds south of Half Moon Bay are 
conformable with the Monterey series below them and with the 
Merced series above them. 
4. That the Monterey series is Miocene; that the Merced 
series on Seven Mile Beach is principally Pliocene, and that the 
tossiliferous beds, transitional between the two, contain a mix- 
ture of Miocene and” Pliocene’ forms. In other wordswuit).a 
line were drawn between the Miocene and the Pliocene it would 
not come at the top of the Monterey series, as usually defined, 
but from one hundred to several hundred feet higher —some- 
where in the fossiliferous beds. 
It would be difficult, if not impossible, to draw the line 
between the two ages, as it would be largely governed by indi- 
vidual inclinations. Accordingly the writer prefers simply to 
call them the Transition beds of the Merced series, grouping 
them with the Merced series because lithologically they are simi- 
lar to the predominating rocks of that series. 
* Univ. of Cal., Bull. Dept. of Geol., L., 143. 
