The concept of an inner (miogeosynclinal) belt and an outer (eugeosynclinal) belt in the Paleozoic of the Great Basin is useful in a broad general way. It is impossible, however, to draw a well marked dividing line between the two troughs not only for the Paleozoic as a whole but also for the individual systems. It seems certain that no sharp uplift or even linear group of uplifts ever separated the troughs and that it is fruitless to search for sharply defined edges for either trough.

The miogeosynclinal sediments are chiefly carbonates with minor shales and sandstones, the eugeosynclinal sediments are highly siliceous types such as chert, arkose, argillite, tuff, and black shale. Tongues of carbonate rock extend westward into the eugeosyncline, and, conversely, beds of black shale and chert are found in the miogeosyncline.

On the basis of aggregate lithologic aspect the western edge of the Rocky Mountain geosyncline has roughly the following course through the Great Basin: commencing near Burley, Idaho, thence to the northwestern corner of Utah, continuing successively through Wells, Cortez, Manhattan, and Goldfield, Nevada, and ending near Owens Lake, California.

Cover of Tertiary sediments and volcanic derivatives is thick and extensive over much of the eugeosyncline and unless deep wells are drilled there the exact nature of the older rocks may never be known.

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