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THE JOURNAL OF GEOLOGY.

But first of all I must define my subject. A mountain range is a single mountain individual—born at one time (monogenetic) i.e., the result of one—though it may be a prolonged—earth-effort; as contra-distinguished on the one hand from a mountain system which is a family of mountain ranges born at different times (polygenetic) in the same general region; and on the other from ridges and peaks which are subordinate parts—limbs and organs—of such a mountain individual. Now a theory of mountains is essentially a theory of mountain ranges, as thus defined. In all that follows, therefore, on the subject of mountain structure and origin, we refer to mountain ranges.

STRUCTURE OF MOUNTAINS.

The origin of mountains is revealed in their structure. We must, therefore, give briefly those fundamental points of structure on which every true theory of origin must be founded.

1. Thickness of Mountain Sediments.—The enormous thickness of mountain strata is well known, but it is impossible to overstate its fundamental importance. We therefore give some striking examples. The Palæozoic rocks involved in the folded structure of the Appalachian, according to Hall, are about 40,000 feet thick. The Palæozoics and the Mesozoic in the Wasatch, according to King, are about 50,000 feet thick. The Cretaceous alone, in the Coast Range of California near the Bay of San Francisco, according to Whitney, are 20,000, and in Shasta county, according to Diller, are 30,000 feet thick. The Mesozoics and Tertiaries of the Alps, according to Alpine geologists, are 50,000 feet.[1] The upper Palæozoic and Mesozoic of the Uinta, according to Powell, are 30,000 feet. These are conspicuous examples, but the same is true of all mountains.

It might be objected that these numbers express the general thickness of the stratified crust everywhere—only that in mountains the strata are turned up and their thickness exposed by erosion. But this is not true. For in many cases the strata may be traced away from the mountain; and in such cases they always thin out as distance increases. For example, the 40,000 feet of

  1. ↑ Judd: Volcanoes, p. 295.
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