172 STUDIES FOR STUDENTS 



All these phenomena are explained by landslides descending 

 from the steep slopes of Old Red Sandstone hills or mountains into 

 the quiet waters of a Jurassic fjord or ria and depositing on the 

 even-layered silts the tumultuous beds of breccia. The debacle 

 would sweep up rounded pebbles from the beaches, tear corals 

 from their bases, crush shell banks in the estuary, and mingle their 

 debris with the fragments of the slides. That earthquakes were 

 the cause of these great rock-falls is suggested by the contemporary 

 sandstone dikes found in the district. 1 



It may be added, if only in illustration of the diverse interpre- 

 tations held of breccias, that Murchison 2 described these breccias 

 as due to crush incident to the upheaval of neighboring granite. 

 Blake 3 argues the deposit of an ice foot. Huddleson, in the dis- 

 cussion of Blake's paper, postulates, ocean currents strong enough 

 to tear up masses of corals and to gather and distribute old shore 

 accumulation of talus, although ocean currents, even if powerful 

 enough to transport the immense blocks of the breccia without 

 wear of edges, are not so paroxysmal as to heap them in the midst of 

 the fine silts of quiet water. Judd 4 recognizes the cataclysmic nature 

 of the formation and suggests very tentatively river-floods of the 

 most violent character. Yet the floods of a river cannot be expected 

 so to maintain their energy on entering the ocean as to deposit 

 their bottom load in water of considerable depth and to mingle it 

 with detritus torn from the ocean floor. The momentum of the 

 rock-fall would seem to be the only force capable of the work, and 

 this origin is advanced by Woodward 5 and by Teall 6 in the dis- 

 cussion of Blake's paper. 



Raft breccias. — In breccias of this class the fragments have been 

 transported in such a way as to escape wear en route. Angular 

 fragments of such soft rocks as shale and talcose, schists and lime- 

 stone, brought unworn from distant sources, prove that the car- 

 riage was upon the surface of the ocean, and not by wave and 

 current along the ocean floor. Further evidence of surface trans- 



1 H. B. Woodward, Quar. Jour. Geol. Soc. London, LVIII (1902), 206. 



2 Transactions Geol. Soc. London, Ser. 2, Vol. II, Part II, p. 293. 



3 Op. cit. 4 Op. cit. s Op. cit. 

 6 Quar. Jour. Geol. Soc. London, LVIII (1902), 205. 



