PRE-CAMBRIAN ROCKS OF NEWFOUNDLAND 477 



is an open question. What little evidence there is seems to point 

 toward a period of batholithic and dike intrusion at a time succeed- 

 ing the period of deposition of the Conception series and preceding 

 that of the Signal Hill series. No dikes whatever were found intrud- 

 ing the Cambro-Ordovician beds in the region of Conception Bay, 

 but in the vicinity of Trinity Bay infrequent dikes of a distinctive 

 character do intrude these beds and undoubtedly belong to a 

 younger period of intrusion, possibly that of the Taconic revolution, 

 or related to the period of vulcanicity represented by vast, flows 

 of lava in the Notre Dame Bay region about fifty miles to the north- 

 east. This period of intrusion during the pre-Cambrian corresponds 

 to a similar period of intrusion of granite into the volcanic series 

 of the Blue Ridge of Maryland and Virginia described by Keith 

 (1892), or the Proterozoic batholiths of the Lake Superior region 

 described by Allen (i9i5,p. 717) which he suggests are post-Middle 

 Huronian or pre-Upper Huronian. 



Geographically, chemically, and physically the volcanics, 

 plutonics, and dikes • of the Conception Bay region appear to be 

 genetically allied. Thus the gabbros, quartz-bearing gabbro, 

 hornblende porphyrite with quartzose groundmass, granodiorite, 

 quartz syenite, biotite granite, granophyre, and aplite form a series 

 of rocks with an ascending silica ratio, belonging to the same 

 geographical area and very probably to the same period of intrusion. 

 Again, the analyses of the Holyrood granite, rhyolite porphyry near 

 Holyrood, and a rhyolite flow from the Avondale volcanics near 

 Manuels, constituting a batholith, a volcanic neck, and a volcanic 

 flow respectively, show but slight variation in composition, although 

 the texture varies widely. The chemical differences between the 

 granite on the one hand, and the rhyohte porphyry and rhyolite on 

 the other, are the usual relative increase in silica and loss of alumina, 

 lime, magnesia, and iron in the volcanic facies. Again, the plutonic 

 gabbros, a volcanic plug of plagioclase basalt near Manuels, and 

 the Blue Hills basalt flows may be cited as textural facies of a similar 

 basic magma. The repeated recurrence and abundance of intru- 

 sions of magma of basaltic composition in dikelike form throughout 

 the entire peninsula, in contrast to the restricted local character of 

 the salic intrusions with their associated infrequent dikes, rather 



