94 GEOLOGY OF THE HIGH PLATEAUS. 



even vitrified (tachylite), and where it has been protracted, the resulting 

 rock has taken the granitoid tex,ture — become, in short, diabase. 



Furthermore, instances of Palaeozoic 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 Regaud, 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 pos- 

 sess all the. essential characters of true lavas. The occurrence of Ter- 

 tiary 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. G. 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 Mount- 

 ains of Colorado we find laccolitic masses of quartz-porphyry graduat- 

 ing into granite porphyry and porphyritic granite. The age of these in- 

 trusions 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 1 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. 



