290 IRVING. 



the Algonkian series, and was marked by the widespread eruption 

 of basic rocks, now represented by the amphibolites, that occur 

 in such abundance throughout the metamorphosed areas. The 

 second was not until the entire series of sediments between the 

 Algonkian and the upper Cretaceous had been deposited, and 

 was characterized by a highly alkaline series of intruded rocks. 

 These two periods of eruptive activity are separated by such a 

 vast interval of time that it is improbable that there should exist 

 any genetic relationships between the rocks peculiar to them. 



Pre-Cambrian Eruptives. 



That the rocks of this first period were extremely basic is 

 shown by their petrographic and chemical characters. Further^ 

 it seems probable they were intrusive or even plutonic be- 

 cause : 



1st : Many of them still preserve the typical diabasic granular 

 texture. 



2d : Many of the more massive varieties show the granitoid 

 texture of gabbro. 



The rocks are too much altered to afford other than these 

 very general conclusions. 



Post-Cretaceous Eruptives. 



The rocks belonging under this head may be differentiated 

 into the following : 



1. Mica-diorite-porphyry or Hornblende-mica-andesite. 



2. Diorite-porphyry or Hornblende -andesite. 



3. Tonalite- or Quartz-mica-hornblende- diorite. 



4. Quartz-porphyry or Rhyolite. 



5. Quartz-porphyrite or Dacite. 



6. Phonolite. 



7. Quartz-aegirite -porphyry or Grorudite. 



8. Augite-vogesite. 



Between the first five varieties there are all gradations. The 

 hornblende-mica-andesites contain at times so much orthoclase 



