The distribution of Mesozoic rocks in the United States is fairly well known. Many individuals doing field work, either independently or for State or Federal surveys, have collected data on outcrops and underground relationships of these rocks during the last hundred years or more. As a result of the publication of their findings, approximately 80 per cent of the areal distribution is known and generally accepted.

Many excellent palegeographic maps have been constructed which show the distribution either by systems or by subdivisions of systems. Outstanding among such maps are those of Charles Schuchert and of Bailey Willis. Recently, C. H. Crickmay published an exhaustive study of the Jurassic rocks of North America, in which he included 13 maps, each showing the distribution during an epoch of that period.2 As those maps are based on a detailed knowledge of faunas and floras, they must be considered authoritative and . . .

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