BY H. I. JENSEN. 551 



rocks of the Sawtooth Mountains include tinguaite, bostonite, 

 paisanite, etc. The lavas are chiefly phonolitic. 



The age of the Texas intrusions is Post-Cretaceous, and pro- 

 bably as late as Eocene or Oligocene. Cretaceous and Laramie 

 sediments overlie in this region very ancient Palaeozoic and 

 metamorphic rocks. 



From Oklahoma in the Headquarters Mountains, a low range 

 at the extreme west border of the Wichita Range, alkaline granite 

 containing hastingsite and riebeckite pegmatites have been 

 recorded by F. Rogers Stanford.* 



Numerous trachyte areas occur in the Colorado, Utah, and 

 Uintah plateaux, but the petrology of this region has escaped my 

 notice, and I do not know how far these rocks can be termed 

 alkaline or not. Their position, as flows and sills of Post-Creta- 

 ceous age along Tertiary fault-lines in a greatly fractured plateau, 

 is analogous to that occupied by the alkaline rocks in Madagascar 

 and Abyssinia, but the western and south-western portions of 

 the United States have been subjected to continuous sedimenta- 

 tion from the Carboniferous period to the Middle Tertiary, and a 

 complete series of Mesozoic sediments is frequently met with in 

 perfect conformity. Earth-movements in the south-western parts 

 of the United States have been mainly vertical movements caused 

 by trough-faulting, folding being limited to special localities 

 where a fragment which has subsided more rapidly than adjoin- 

 ing fragments has been crushed between them. The conditions 

 under which the lavas of Utah and Colorado have been produced 

 are therefore closely analogous to those accompanying the East 

 African alkaline extrusions. 



One of the most important alkaline areas in North America is 

 that of Arkansas, described by J. Francis Williams.* The rocks 

 described comprise pulaskite, nepheline syenite, tiuguaites, 

 quartz syenite pegmatites, fourchite, monchiquitesand peridotites. 

 They occur as dykes and laccolites intruding the rocks of the 



* Journ. of Geol. xv. 3. 

 * Ann. Report Geol. Survey of Arkansas, 1890, "Vol.ii. 



