306 



and this composite character of the rock is thought, by the 

 author, to be a characteristic that may have been favourable 

 for its production. The, hard epidiorite forms the chief 

 brecciated fragments, while the plasticity of the composite 

 limestone has led to its being squeezed and mashed so as to 

 form the main groundmass. 



INTRAFORMATIONAL CONGLOMERATES. 



This name was given by Dr. C, D. Walcott to -certain 

 broken and conglomeratic beds that occur in the Cambrian 

 and Ordovician rocks of North America. (13) 



A typical section, exposed on the eastern shore of the 

 Hudson, shows thinly-bedded limestones, containing an 

 Olenellus fauna, which, in both the upper and lower portions 

 of the section, are undisturbed, but are separated by a con- 

 glomeratic band that is from two to six feet in thickness. The 

 conglomerate consists of limestone pebbles, more or less 

 rounded by attrition, and contains fossils that are identical 

 with those that occur in the undisturbed parts of the section, 

 and are set in a calcareous matrix. Other examples of a 

 similar kind are described from localities situated in Canada 

 (Cambrian and Ordovician) ; Vermont and New York 

 (Passage-beds between the Cambrian and Ordovician) ; 

 Pennsylvania (Lower Cambrian) ; Virginia (Cambrian and 

 Cambro- Silurian) ; and Tennessee (Cambrian). As to the 

 origin of these conglomerates, Walcott thinks that the 

 calcareous muds, represented by the beds, were solidified soon 

 after their deposit, and were raised in ridges or domes above 

 sea level, and that these latter were subjected to the action 

 of sea-shore ice, if present, and the debris worn from the 

 exposed ridges was deposited in the intervening depres- 

 sions. (14^ 



INTRAFORMATIONAL BRECCIAS. 



According to Grabau(i5) ''these are contemporaneous 

 phenomena, formed as one of the sequential divisions of a 

 single rock series. They are almost wholly confined to 



(13) ''Palaeozoic Intraformational Conglomerates," by C. 

 D. Walcott, Bull. Geolog. Soc. of Am., vol. v. (1894), pp. 191-198, 

 pis. v.-vii. The same paper, with additional plates, was reprinted 

 at the end of Walcott's paper on "The Cambrian Rocks of 

 Pennsylvania," Bull. U.S. Geolog. Sur., No. 134 (1896), pp. 34-40, 

 pis. x.-xv. 



(14) Sir William Dawson also thought that, in the case of the 

 Canadian examples, ice agency was the cause. See Dawson, "On 

 the Eozoic and Palaeozoic Rocks of the Atlantic Coast of 

 Canada," Quar. Jour. Geolog. Soc., vol. xliv. (1888), p. 809. 



(15) "Principles of Stratigraphy," 1913, p. 529. 



