Franciscan and Knoxville rocks are widely distributed and attain a great thickness in the Coast Ranges of California and southwestern Oregon. In California they crop out in a known area of at least 14,000 square miles and are estimated to underlie an additional 16,000 square miles, thus covering approximately one-fifth of the total area of the state. Even so, this is probably less than half of their original extent. The geosyncline in which they were deposited was more than 700 miles in length and probably more than 100 miles in width. Owing to removal of the higher beds by erosion, fully 90 per cent of the present outcrops are Franciscan, as that term is usually used. The name Franciscan implies a lithologic assemblage characterized by arkosic sandstones, radiolarian che ts, pillow lavas, basic and ultrabasic intrusives, and pneumatolytic metamorphics (glaucophane and related schists). Unfortunately, the Franciscan has been made a catch-all for several wholly unrelated formations in southern California and the Sierra Nevada. If many of the correlations proposed were correct the name Franciscan would have neither stratigraphic nor lithologic significance and should be abandoned; it would be practically synonymous with the lower Mesozoic. Such unjustifiable correlations have unnecessarily confused an already complex problem. Evidence is presented to show that the Franciscan is not a catch-all of uncertain age and of no stratigraphic significance but, together with the Knoxville, occupies a comparatively short space of geologic time (Tithonian, Upper Jurass c) and is of great stratigraphic significance.

The term Knoxville is used here for those sediments and volcanics above what is commonly regarded as Franciscan and below the Lower Cretaceous. As usually described, the Knoxville consists of a thick series of shales, with minor amounts of sandstone and conglomerate. This is an adequate description of the bulk of the Knoxville in many localities but it ignores certain important rocks in some localities and entire sections in other regions. The Stanley Mountain region in southern San Luis Obispo County may be cited as an example. Here the lithologic composition is both Franciscan and Knoxville, arkosic sandstones, dark clay shales, pillow lavas, radiolarian cherts, and basic and ultrabasic intrusives, but the fauna is definitely that of the thick section of Knoxville shales in other pa ts of the state. Furthermore, pillow lavas, radiolarian cherts, and basic and ultrabasic intrusives are common in the Knoxville in many regions. Although there is a lithologic difference between Franciscan and Knoxville in many areas this difference is not universal and the types commonly regarded as Franciscan are in many places found in what must be called Knoxville on the basis of the fauna.

This common lithologic similarity results from the fact that the widespread volcanism that in most places began well down in the Franciscan persisted for a longer time in some regions than in others. Thus volcanism, with the attendant formation of radiolarian cherts, continued practically to the close of the Jurassic in some localities; at the same time, dark clay shales usually regarded as characteristic of the Knoxville, and with a typical Knoxville fauna, accumulated in areas free from volcanism. Volcanism neither began nor ended at the same time everywhere nor was it of equal intensity throughout the late Jurassic geosyncline in which the Franciscan and Knoxville accumulated.

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The profound unconformity between the Franciscan and the Knoxville, reported by many writers, is a myth; it is unsupported by field evidence. The Lower Cretaceous, which is lithologically similar to the sedimentary part of the Knoxville, is commonly confused with the latter and the unconformity, or disconformity, at its base has been the chief cause of the reported unconformity at the base of the Knoxville.

The Knoxville is regarded by the writer as an upper phase of the Franciscan. Since these names have become fixed in the literature, and since it is not desired to introduce new terms, it is suggested that the name Franciscan-Knoxville group be used for the entire sequence. No confusion should be caused by such usage; local formational names may be given as the result of adequate mapping.

The Franciscan-Knoxville sequence accumulated in a geosyncline that came into existence as a result of the Nevadan orogeny (Portlandian) that strongly folded the earlier Mesozoic sediments of the present Sierran region. As the ancestral Sierra Nevada emerged, a long and broad trough was formed and the site of the present Coast Ranges was flooded for the first time in the Mesozoic. At the same time a high and rugged land mass came into being, or an already existent land mass was greatly uplifted, west of the present coast line. The character of the sediments in the Franciscan-Knoxville group and the direction from which they were derived strongly support the concept of this western land mass; seismologic evidence also lends support. In the great trough east of this rugged range and wes of the emerging Sierra Nevada the Franciscan-Knoxville rocks accumulated; this marks the beginning of the earliest readily decipherable history of the present Coast Ranges. During the early history of the geosyncline, arkosic sandstones accumulated rapidly; geosynclinal sinking kept pace with deposition. As the land mass was worn down, the stream gradients decreased, and predominant mechanical disintegration gave way to predominant chemical decomposition, a gradual change took place in the character of the sediments supplied. The arkosic sandstones pass upward into the dark clay shales characteristic of the Knoxville in many localities. Thus the Franciscan and the Knoxville form a normal depositional cycle. This cycle is, of course, complicated by widespread volcanism and the formation f chemical sediments such as the cherts. These may appear at almost any horizon but they are most abundant in the upper part of the Franciscan. Thus the Franciscan-Knoxville group is not only characterized by certain distinctive lithologic types but also by the sequence in which these appear.

Local disturbances took place on the margins of the geosyncline during the accumulation of the thick Franciscan-Knoxville group but at no time was the entire trough affected. There are local discontinuities in deposition but there is no record of widespread diastrophism until the beginning of the Lower Cretaceous. The geosynclinal margins may have been modified but deposition was continuous over most of the trough.

There is a very general misconception regarding the stage of alteration of the Franciscan due in large measure to its commonly disordered appearance and the presence of schists. The chief causes of the disordered appearance in many localities are the heterogeneous nature (flows and chemical sediments interbedded with normal clastics and numerous intrusives) and the many periods of diastrophism suffered by the Franciscan during the Cretaceous and Tertiary. The Franciscan has been folded and faulted repeatedly and the sediments have been crusted against the more resistant volcanics and intrusives, resulting in extensive shearing and slickensiding in many regions. Where volcanics and intrusives are scarce or absent the sediments have been little altered except by depth of burial.

The glaucophane and related schists can not be regarded as evidence for extensive regional metamorphism as they are confined to the borders of intrusives and are clearly the result of local pneumatolytic contact action.