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The Origin of Some Chromite Deposits in the Pacific Coast Region:
Chromite deposits have long been regarded as products
of magmatic segregation. As additional data have become
available, other processes have been invoked to account
for the observed facts, The writer, in the employ of the
U.S. Geological Survey, has visited many of the known
chromite localities in the western part of the United
States and has collected much field evidence bearing on
the problem of the origin of chromite.
Laboratory data furnishes much information which is
not 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 connection
with 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 thought
to approach equilibrium, presumably consisting of a layer
of olivine and small amounts of chromite in the base of
the magma chamber, succeeded by a layer of chromite and
topped by a large mass of olivine with small amounts of
chromite; a thin layer of pyroxene may cap the mass if
any is present. This ideal condition is seldom attained in nature, except in the stratiform deposits, and the degree of aberration from equilibrium conditions determines
the characteristics of the ore deposit formed.
The origin of the various chromite deposits may be
likened to quenching practice used to determine the constituents of a high temperature melt or liquid at various
stages during its cooling cycle. Each chromite deposit
is examined for evidence indicating at what stage in the
process of cooling or crystallization of the ultramafic
magma its development was arrested by relatively rapid
cooling or quenching. Following physico-chemical principles, beginning with the ultramafic magma in a fully liquid state, many of the difficulties in understanding the
formation of various types of deposits are partially explained.
The Biostratigraphy of Glycymeris veatchii in California:
The stratigraphy and correlation of upper Cretaceous
rocks comprise one of the major problems of Pacific coast
geology. It is particularly significant as the search
for petroleum penetrates into progressively older rook
formations in California. As a step toward clarification
of the problem this paper attempts to indicate the possible
evolution and variation in a single fossil molluscan species
during 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 cretaceous
localities used in this paper is presented, and systematic
description of the species and varieties described included.
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Fecha de contribución: 06-jul-2018
* Smith, Clay Taylor (1943) The Origin of Some Chromite Deposits in the Pacific Coast Region [major thesis]. The Biostratigraphy of Glycymeris veatchii in California [minor thesis]. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:07022018-144808790