CHERT FORMATIONS OF NOTRE DAME BAY 589 



from one quarter of an inch to several inches. As many as thirty- 

 bands have been counted within three feet. The bedding of the 

 chert is somewhat irregular, and this the authors ascribe to motion 

 before its consolidation. 



Even more striking are the Ordovician radilaria-bearing cherts 

 of southern Scotland.^ A continuous belt of Ordovician and 

 Silurian rocks some 40 miles in width extends across the Southern 

 Highlands of Scotland from coast to coast. The Ordovician rocks 

 are best exposed in the northern part of this belt, particularly at 

 the western extreme in the Girvan area. The lowest Ordovician 

 is represented by a series of volcanics consisting of agglomerates, 

 tuffs, and lava flows showing pillow structure to great perfection. 

 In places a little chert is associated with the uppermost pillow lavas. 

 These are overlain by a few feet of black graptolitic shale which 

 fixes their age. Succeeding these is a series of radiolarian cherts and 

 "mudstones," some 70 feet in thickness, which occurs sporadically 

 over an area of about 2,000 square miles. The cherts are often 

 interbedded with tuffs. The bedding is irregular, and in places a 

 bed of chert may have a botryoidal upper surface. Concerning the 

 cherts of the Girvan area the authors say: 



The volcanic series passes upwards into red, green, and gray cherts which 

 are interstratified with tuffs and breccias that clearly overlie the Middle 

 Arenig band of black shales. Indeed the cliff sections leave no room for doubt 

 that the radiolarian cherts were deposited contemporaneously with the volcanic 

 eruptions, for not only are they intercalated with the breccias, but the latter 

 likewise contain fragments of organic chert with radiolaria, which must have 

 solidified on the sea floor before its disruption by the explosion.^ 



The authors, following Hinde, regard these cherts as radiolarian 

 oozes. 



The Middle Ordovician or Llandeilo is conformable with the 

 Arenig. It consists of cherts and "mudstones" with some inter- 

 bedded tuffs and breccias. ''In a few exceptional localities [in the 

 northern part of the belt] the cherts are apparently intercalated 

 with graywackes in such a way as to preclude the supposition that 

 the relationship is due to faulting," which would indicate that if 



' B. N. Peach and J. Home, The Silurian Rocks of Britain, Vol. I, Mem. Geol. 

 Surv. United Kingdom, 1899. 



^ Ibid., p. 40. 



