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THE POPULAR SCIENCE MONTHLY. 



IIow Monntains are made. In a paper 

 by Prof. Joseph Le Conte, read before the 

 National Academy of Sciences in April, and 

 since published in the American Journal 

 of Scie?ice, the formation of mountains is 

 explained by tbe action of horizontal press- 

 ure resulting from interior contraction of 

 the earth. The author considers all the 

 principal types of mountain-structure, and 

 appears to account for them very satisfac- 

 torily by this theory of horizontal pressure. 

 It would be impossible, within the limits of 

 a Miscellany article, to give an intelligible 

 outline of the entire argument, and we must 

 content ourselves with a synopsis of the au- 

 thor's remarks on the formation of moun- 

 tains of a single anticlinal fold the sim- 

 plest conceivable mountain-structure. Here 



Fig. 1. Ideal Section of Simple Anticlinal 

 Fold (unsysimetrical). 



the deeper strata are thickened and swelled 

 upward by the horizontal pressure, while 

 the upper strata are raised into a vault with 

 little or no thickening, or may even be 

 thinned and broken by tension. The vault 

 is nearly always unsymmetrical, the yielding 

 being greater on one side than on the other. 

 In such cases a great fissure and slip is apt 

 to occur on the steeper side, as shown in 

 Fig. 2. Perhaps the best illustration of a 



Fig. 2. The same, with the strata Assured and 

 faulted. 



range of mountains of this simple type is the 

 Uintah Mountains. A cross-section of this 

 range shows a prodigious fault of 20,000 feet 

 on the northern or steeper side of the origi- 

 nal fold. If the crust of the uprising region 

 be extremely rigid, the vault, instead of being 

 forty or fifty miles across, as in the Uintah 

 Mountains, may be one hundred or several 

 hundred miles across ; we have then a. great 

 plateau. And since an arch of such span, 



whether filled or not beneath with fused or 

 semi-fused matter, cannot sustain itself, 

 such elevated plateaus are peculiarly liable 

 to fissure by breaking down of the arch, 

 and to slips by gravitative adjustment of 

 the broken parts. The result is conspicu- 

 ous escarpments or conspicuous mountain- 

 ridges in the general direction of the axis 

 of the plateau. Such, according to the au- 

 thor, is the origin of the north and south 

 escarpments of the plateau-region of the 

 Colorado, described by Powell, and of the 

 north and south monoclinal ridges in the 

 Basin-region, described by Gilbert and How- 

 ell. Again, a monoclinal fold may be mod- 

 ified by inetamorphism. This, says the au- 

 thor, is especially apt to be the case if the 

 strata be very thick and the fold narrow 

 and high ; that is, if the compression in a 

 given space, and therefore the heat of com- 

 pression, be very great. If, now, such a 

 sharp fold, metamorphosed in its deeper 

 strata along the line of greatest compres- 

 sion, be subjected to profound erosion, it 

 forms a common type of mountain, viz., one 

 consisting of a highly-metamorpliic axis, 

 flanked on either side by tilted strata cor- 

 responding to each other. 



The Differences between Atlantic and 

 Pacific Forests. The differences between 

 the Atlantic and Pacific forests of the United 

 States are very striking in many respects. 

 Prof. Asa Gray, in a recent lecture, presents 

 a long list of Atlantic forest-trees that are 

 either not at all or but feebly represented 

 on the Pacific slope. For instance, the Pa- 

 cific forest has no magnolias, no tulip-tree, 

 no papaw, no linden or basswood, and is 

 very poor in maples ; no locust-trees, nor 

 any leguminous tree; no cherry-tree large 

 enough for a timber- tree; no gum-trees, 

 nor sorrel-tree, nor kalmia ; no persimmon ; 

 not a holly ; only one ash that may be 

 called a timber-tree; no catalpa, or sassa- 

 fras ; not a single elm nor hackberry ; not 

 a mulberry, nor planer-tree, nor maclura ; 

 not a hickory nor a beech, nor a true chest- 

 nut nor a hornbeam ; barely one birch-tree, 

 and that only far north, where the differ- 

 ences are less striking. As to coniferous 

 trees, however, the only missing type is our 

 bald cypress, the so-called cypress of our 

 Southern swamps. " But as to our ordinary 



