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Show 94 GEOLOGY OF THE HIGH PLATEAUS. even vitrified (tachylite), and where it has been protracted, the resulting rock has taken the granitoid textureâ€"become, in short, diabase. Furthermore, instances of Palseozoic trachyte are not wanting. In the Laurentian rocks of Canada they are, according to Dr. T. Sterry Hunt,* very abundant and extensively displayed. At Brome and Shefford they occupy two areas of twenty, and nine, square miles, respectively, and their period of eruption must have been soon after the Quebec epochs At Yamaska a micaceous trachyte occurs differing from the foregoing, and at Chambly and Eegaud, a porphyritic trachyte. The island of Montreal offers a great variety of trachytic rocks, some of which, according to Dr. Hunt, cannot readily be distinguished from the trachyte of Puys de Dome. At Lachine a phonolite is also mentioned as associated with trachytic dikes. Thus we do find among Pre-Tertiary eruptives rocks which possess all the essential characters of true lavas. The occurrence of Tertiary granitoid rocks is probably less common. Still they do sometimes occur. True porphyries of Tertiary age are much more frequent. Those intrusive masses, to which Mr. Gr. K. Gilbert has given the name of laccolites, are in every sense porphyries. Most of them, however, belong to the non-quartziferous division of felsitic porphyry, and are distinct from the common elvanite or quartz-porphyry. But in the Elk Mountains of Colorado we find laccolitic masses of quartz-porphyry graduating into granite porphyry and porphyritic granite. The age of these intrusions is not accurately known, though it is certain that they are Post-Cretaceous. Laccolitic rocks of trachytic and rhyolitic constitution seem to be tolerably abundant throughout the mountain regions of the West. Nevertheless, the fact remains that the Pre-Tertiary eruptives are on the whole preeminently granitoid or porphyroid in texture, while the Tertiaries are as decidedly volcanic. It seems, therefore, at first as if a correlation existed between age and texture. Forthwith arises the inquiry, what is the significance of .that relation ? To this question it seems to me that Von Cotta has given a very satisfactory answer, which may be summarized as follows. The eruptive magmas of Tertiary time did not differ at the time of eruption in any material respect from those of older epochs, any more than * Geology of Canada, 1863, p. 656. |