Origin of some of the siliceous Miocene rocks of California:
The upper Miocene marine section in California is characterized in many places by the presence of unusually large amounts of highly siliceous rocks. These can be roughly divided into two classes: the lithified chert-like beds which are characteristic of, and generally largely confined to, the lower part of the section, and the unconsolidated diatomites and diatomaceous and radiolarian silts which are commonly found overlying them. The chert-like beds have been referred to in California geological literature by a variety of names, including cherty shales, platy shales, siliceous shales and cherts. The merits of these, and some other designations are discussed in the section on nomenclature and it will suffice to say here that the writer has decided to use only the terms chert and cherty shale. In addition to the siliceous rocks the lower part of the Upper Miocene or the immediately underlying part of the section frequently contains considerable amounts of volcanic material, both extrusive and intrusive in nature.
The Stratigraphic Position of Some of the Diatomite Horizons in the Los Angeles Basin:
In most places in the Los Angeles Basin where a complete section of upper Miocene rooks is exposed there exists a mappable unit which is composed largely of soft, white, highly diatomaceous silt. This unit is commonly referred to as the diatomite. It varies considerably in thickness and in its relationship to other rook types, but the constancy of its lithologic characteristics considered in conjunction with their unusualness suggests that the unit bas stratigraphic significance. Since, however, structuraland physiographic conditions make it impossible to trace the diatomaceous deposits continuously around the Los Angeles Basin, the only method of determining the age relationships of various isolated areas of these rooksis the paleontological one.
