576 EDWARD SAMPSON 



in places highly fossihferous. Volcanic rocks are almost entirely- 

 absent in the known Silurian. No chert has been found in the 

 Silurian or in the upper part of the Ordovician. 



Intrusive igneous rocks are abundant and varied and greatly 

 complicate the structure of the region, but as they have no bearing 

 on the present problem they need not be discussed. 



DISTRIBUTION OF CHERTS 



The area in which the cherts occur on the coast is clearly limited, 

 as they are confined to that portion within which the pillow lavas 

 are found. The pillow lavas are thick conspicuous formations 

 which in this part of the Newfoundland coast do not occur north 

 of Cape St. John nor east of New World Island. Within Notre 

 Dame Bay they are exposed at many places. 



VARIETIES OF CHERT AND ASSOCIATED ROCKS 



GENERAL STATEMENT 



Wherever the cherts have been observed, with the exception of 

 only one occurrence, to be considered later, they are intimately 

 associated with volcanic rocks. Three general modes of occurrence 

 are recognized: first, in the spaces between pillows; second, as 

 heavy beds of jasper associated with acid tuffs; and third, as thin 

 beds of green chert, associated with tuffs, in sections which are 

 made up principally of fragmental material and shale. The first 

 type, interstitial, occurs in the oldest pillow lava series of probable 

 Cambrian age; the second type, heavy beds with tuff, is probably 

 also of the same age ; and the third type, which consists of thin beds, 

 is in several widely separated localities in close proximity to the 

 black graptolitic shale above referred to and is probably of the same 

 age, Llandeilo. Pillow lavas are found in some of these sections 

 though not in immediate association with the chert beds. 



The flows of pillow lava, throughout the region, are com- 

 monly from 50 to several hundred feet thick. Individual pillows 

 vary greatly in size, being sometimes as much as 10 feet in diameter, 

 but the average greatest cross-section is about three or four feet. 

 They usually approach spherical form, but quite commonly bolster- 

 like masses are found. They are exceedingly massive and very 



