PETROLOGY IN AMERICA. 475 



in the present communication any notice of the varied series 

 of Tertiary effusive rocks, as we have already passed over 

 the equally interesting ancient lavas and tuffs of the eastern 

 states ; and we go on to notice an important memoir by 

 Whitman Cross (28) on a widely distributed type of in- 

 trusive rock. It was about twenty years ago that Peale, 

 Holmes, and Gilbert investigated the nature of the large 

 lenticular masses of igneous rocks which occur in various 

 parts of the Rocky Mountains of Colorado, and have given 

 rise to numerous isolated groups of mountains farther west, 

 in Utah and Arizona. To intrusions of this form, uplifting 

 the strata above them in the fashion of a dome, Gilbert 

 gave the name laccolite, and the highly symmetrical 

 examples described by him in the Henry Mountains in 

 Utah have remained the type of this kind of occurrence. 

 But little petrographical examination was made at that 

 time of the rocks constituting these laccolites, which were 

 generally referred to as trachytes. In 1887 Cross pointed 

 out that this name was not well chosen, and that trachyte 

 in the strict sense is certainly not a common rock-type in 

 the region. In his recent paper, besides adding consider- 

 ably to our knowledge of the nature of these laccolitic 

 intrusions and the varieties of form which they take on in 

 different circumstances, he has given a very complete 

 account of the rocks which compose them. The most 

 striking feature of these rocks is their uniformity in chemi- 

 cal, mineralogical, and structural characters throughout a 

 very extensive tract, prolonged northward apparently into 

 the Yellowstone Park and perhaps into Montana. The 

 analyses show variation between certain limits in silica- 

 percentage and in some other particulars with a remarkable 

 constancy in certain other features. The alkalies jointly 

 are always about 6 or 7 per cent., potash and soda being in 

 equal quantities in the rocks of Colorado, while the latter 

 preponderates in those of the plateau groups to the west 

 and in the Yellowstone district. All, or nearly all, the 

 rocks are porphyritic, crystals of plagioclase felspar and 

 ferro-magnesian silicates having been formed prior to the 

 intrusion of the magma. Plagioclase is predominant and, 



