824 



DYNAMICAL GEOLOGY. 



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reason, the areas over the earth's surface that were affected by single 

 movements were of very vast extent. Of this kind were the high- 

 latitude movements of the Quaternary. Besides the downward bend- 

 ing over those higher latitudes in the middle Quaternary, there was 

 another in the warm parts of the oceans — the coral-island subsidence. 

 Both bear the character of the times in the extent of surface involved, 

 and are wholly unlike the mountain-making geosynclinals of earlier 

 time. It is probable that the Pacific coral-island subsidence, or 

 geosynclinal, was the counterpart of the geanticlinal over the conti- 

 nents of the later Tertiary and early Quaternary. 



For a similar reason, fractures and outflows of igneous rocks be- 

 came numerous after the crust had become too much stiffened to bend 

 easily. Great floods of doleryte and trachyte were poured out over 

 the Rocky Mountain slope after the close of the Cretaceous period. 

 The previous plications and solidifications of the strata, involved in 

 the making of the various ranges of mountains on the Pacific side of 

 the continent — the Sierra Nevada and the coast ranges on the west, 

 and the Wahsatch and other mountains on the east — had left the 

 crust firm and unyielding ; and, being too stiff to bend, it broke, and 

 out leaped the fiery floods. 



From the descriptions of the Henry Mountains, of Western Colorado, by Mr. G. K. 

 Gilbert, it appears that mammiform bulgings of the strata (Cretaceous to Subcarbon- 

 iferous), with quaquaversal dip, were made, even to a height of 5,000 feet above the 

 surrounding plateau, by the injection of trachyte into chambers between beds at differ- 

 ent levels in and over the Carboniferous, which chambers were opened, in his view, by 

 the forcibly upthrust liquid rock. (See p. 747). He applies the term laccolite (from 

 Aa«Kos, cistern, and Ai'So? (better laccolith ?) to such injected masses. 



4. Valleys. — The valleys of the world are mostly valleys of denu- 

 dation, and, as has been stated, they are the work chiefly of the fresh 

 waters of the land ; the sea and the winds having contributed little 

 toward the great result. The channels now occupied by the sea, 

 called fiords, and a large part of the bays, besides many straits or 

 channels among off-shore islands, are embraced among the results of 

 erosion by fresh waters. 



There are besides, inter-accumulation depressions, as the previous 

 descriptions have shown. They are regions left relatively low, owing 

 to accumulations either side. Circumstances sometimes favor the de- 

 position of material along lines, or bands, where the region between 

 remains low without any help from erosion. The forming of beaches 

 and drift heaps may result in little valleys or relativety low areas of 

 this kind ; and if the beaches consist of coral or shell sand, the accu- 

 mulation becomes a bed of limestone. Where subsidence is added, 

 the depression is an effect of combined accumulation and subsidence, 

 and has, in some instances (as between coral islands) become thou- 



