326 Proceedings of the Royal Irish Academy. 



LII. PiJELIMINAET NoTICE OF A MeMOIR ON RoCK-JOINTlNG, IN ITS 



EELATION TO PHENOMENA IN PHYSICAL GeOGKAPHY AND PHYSICAL 



Geology. By Peofessok William King, Sc.D., &c. (Abstract by 

 the Author.) 



[Read, Jime 18, 1880.] 



The author in the beginning introduces the principal conclusions (of 

 ■which there are six) which he has arrived at in his "Peport" on the 

 Jointing of Rocks, &c., published in the Transactions of the Royal 

 Irish Academy, 1875, Yol. xxv. ; after which he notices what has lately 

 been published on the subject by Daubree and Sorby, whose remarks, 

 he maintains, are confirmatory of his main conclusions. 



According to the author, jointing is a physical phenomenon, which, 

 constituting lines or zones of weakness in the earth's crust, has per- 

 mitted subterranean disturbances, often accompanied by igneous 

 upbursts, to follow the courses of these zones. Such disturbances 

 have greatly afi^ected rocks possessed of jointing, compressing them at 

 right angles to the course of an axis of disturbance, and bringing the 

 planes of jointing into immediate contact; thus developing slaty 

 cleavage. The same agencies, besides producing enormous disloca- 

 tions or faults, have, by their transgressive action, flexured rocks into 

 mountain chains, and intervening valleys into parallelism with an 

 axis of disturbance. 



Although agencies of the kind have often obliterated jointing, the 

 writer assumes that cleavage planes represent it ; also that the strike 

 of these planes indicates the course or direction pursued by the 

 obliterated jointing, and consequently the system to which this 

 divisional structure belonged. 



He, moreover, regards the coast-lines of continents as a correlative 

 phenomenon ; taking these features to be defined by the great sub- 

 marine slopes which rapidly descend into the abysses of the oceans. 



This last point brings the author in contact with Dana, who 

 contends that the earth's continents have always been continents, or, 

 as the writer puts it, that the present continents, in the main, have 

 been from the earliest geological periods greatly elevated regions, 

 separated by enormously deep depressions. So far, there is some 

 agreement between both authors. Dana, however, contends that our 

 continents have been produced by regional up-bendings and down- 

 bendings of the earth's crust, the former giving rise to continental 

 masses, and the latter to great ocean basins ; whereas the writer, 

 although accepting tbe pre-Cambrian antiquity of the main surface 

 features of the earths crust, maintains that our continental coast- 

 lines are in correlation with enormous faults, which have thrown 

 down the rocks on one side of a dislocation thousands of feet below 

 their corresponding masses on the other side. He furthermore 

 maintains that the general direction of any given continental coast- 

 line has been determined by some system or section of jointing. 



