CHERT FORMATIONS OF NOTRE DAME BAY 577 



rarely show hollow centers. Concentric rings of amygdules are 

 frequently seen and varioHtic structure is common, the variolites 

 increasing in size and number toward the center where they 

 coalesce to form a dense crystalline rock. Variolites are seldom 

 over a centimeter in diameter, and in most cases but a few milli- 

 meters. The variolitic shell ranges in thickness from a few milli- 

 meters to 10 or 15 centimeters or even more. Pahoehoe lava and 

 the various ropy forms so characteristic of subaerial flows are absent. 

 In Notre Dame Bay pillow lavas are probably shown in as great 

 perfection and profusion as in any other region yet described. 



Although recognizing that pillow lavas may form on a small 

 scale on land, the author believes that a review of the literature jus- 

 tifies the conclusion that a thick series of pillow lavas, covering a 

 wide area and free from pahoehoe and massive flows, is in all prob- 

 ability a subaqueous flow, and such an origin is accepted for the 

 pillow lavas with which the cherts in question are associated. 



FIRST TYPE — INTERSTITIAL 



The first type of chert to be considered in detail is that which 

 occurs in the spaces between the pillows. The chert is found 

 in irregular masses conforming to the pillow surfaces and seldom 

 over eight inches in greatest dimension. The chert does not 

 replace the surrounding rock which shows the usual gradational 

 changes through variolitic or amygdaloidal structure to the tachy- 

 litic crust next to the chert. Good exposures of chert in pillow 

 lava may be seen at Round Harbor, the bottom of Snooks Arm, 

 the Betts Cove copper mine and on the southwest shore of the 

 Northwest Arm of Fortune Harbor, 



SECOND TYPE — ^HEAVY BEDS OP JASPER 



The second manner of occurrence of the chert is in heavy beds 

 associated with rhyolitic tuffs and forming part of the old 

 (Cambrian?) pillow lava series. The chert in this association is 

 usually a bright red jasper and in places it is very dark red, due to a 

 considerable manganese content. As the associations are so much 

 the same in widely separated localities, these rocks may form a 

 distinct member of the series. 



