BY H. I. JENSEN. 553 



in the Laurentian which has been intruded. The Montreal 

 syenites were intruded after the Trenton and before the Lower 

 Helderberg Series were deposited, and are therefore of Upper 

 Silurian age. 



W. S. Bay ley* describes an intrusion of basic rock of Palaeozoic 

 age which marginally through assimilation and fusion of the 

 country rocks (the Animikie slates and indurated quartzites) 

 passes into a red rock, a kind of quartz-keratophyre. The latter 

 is occasionally riebeckite-bearing. Strong chemical and mineral- 

 ogical evidence is given in the paper to show that the typical 

 red rock is simply the sedimentary rock fused and recrystallised, 

 and that the connecting links between the red rock and the 

 gabbro are mixtures of the intruding magma and the intruded 

 rock in various proportions. 



In the region of the Lower Mackenzie River and Bear River, 

 Canada, phonolitic mountains rise out of Archaean and Cretaceous 

 formations. The Cretaceous and late Palaeozoic rocks of this 

 region represent mere transient transgressions, and the volcanic 

 rock is Post-Cretaceous. 



From Greenland nepheline-bearing and melilite-bearing rocks 

 have been described by Tornebohm. 



All the foregoing alkaline rocks found along the eastern border 

 of the North American horst, from Arkansas to Montreal, occur 

 in a region which has scarcely undergone any folding since 

 Archaean times. The Palaeozoic sediments are horizontal or 

 nearly so. The intrusive rocks, whether Tertiary or Palaeozoic 

 in age, have been produced under circumstances similar to those 

 existing in the Abyssinian region in the volcanic era. There has 

 been faulting accompanied by vertical movements, and the forma- 

 tion of trough-faults. Such was also the case in the Apache 

 Mountains and the Sawtooth Mountains of Texas. In the Black 

 Hills of Dakota, along the fault which borders the Rocky 

 Mountains on the east in this region, alkaline rocks have also 

 been extruded. 



* " Eruptive and Sedimentary Rocks of Pigeon Point, Minnesota," Bull. 

 109, U.S. Geol. Surv. 



41 



