The Origin of Some Chromite Deposits in the Pacific Coast Region:
Chromite deposits have long been regarded as productsof magmatic segregation. As additional data have becomeavailable, other processes have been invoked to accountfor the observed facts, The writer, in the employ of theU.S. Geological Survey, has visited many of the knownchromite localities in the western part of the UnitedStates and has collected much field evidence bearing onthe problem of the origin of chromite.
Laboratory data furnishes much information which isnot compatible with some of the field data. The temperatures of intrusion of ultramafic magmas, the time-sequence of crystallization of the various constituents, and the degree of liquid immiscibility are a few of the features indicated by field relations which are difficult to reconcile with laboratory data.
Spectrochemical investigations made in connectionwith this research suggest that the minor elements contained in the chromite molecule, namely, nickel, manganese, silver, vanadium, sodium and cobalt, show regular variations in amount when arranged in the time-crystallization sequence proposed in this paper; such differences are thought to represent variations in the conditions of crystallization and deposition.
Each chromite deposit is regarded as having been formed under the influence of several forces. Under ideal conditions the resultant of these forces is thoughtto approach equilibrium, presumably consisting of a layerof olivine and small amounts of chromite in the base ofthe magma chamber, succeeded by a layer of chromite andtopped by a large mass of olivine with small amounts ofchromite; a thin layer of pyroxene may cap the mass ifany is present. This ideal condition is seldom attained in nature, except in the stratiform deposits, and the degree of aberration from equilibrium conditions determinesthe characteristics of the ore deposit formed.
The origin of the various chromite deposits may belikened to quenching practice used to determine the constituents of a high temperature melt or liquid at variousstages during its cooling cycle. Each chromite depositis examined for evidence indicating at what stage in theprocess of cooling or crystallization of the ultramaficmagma its development was arrested by relatively rapidcooling or quenching. Following physico-chemical principles, beginning with the ultramafic magma in a fully liquid state, many of the difficulties in understanding theformation of various types of deposits are partially explained.
The Biostratigraphy of Glycymeris veatchii in California:
The stratigraphy and correlation of upper Cretaceousrocks comprise one of the major problems of Pacific coastgeology. It is particularly significant as the searchfor petroleum penetrates into progressively older rookformations in California. As a step toward clarificationof the problem this paper attempts to indicate the possibleevolution and variation in a single fossil molluscan speciesduring upper cretaceous and early Tertiary time.
The results of the present study are three-fold:(l) The evolution of Glycymeris veatchii has been traced.(2) The number of ribs on a valve is found to vary inversely with the geologic age of the specimen. (3) Two new varieties, Glycymeris veatchii reddingensis, and G. veatchii anae, and one new species, G. maganosensis, have been recognized, and G. major is shown to be an invalid species.
A tentative correlation of the upper cretaceouslocalities used in this paper is presented, and systematicdescription of the species and varieties described included.
