SUPPLEMENTARY NOTE A. 107 



&UPPLEMENTARY NOTE A, 



IN the abstract referred to in a previous page I first broached 

 the hypothesis of cyclical vertical movements on a regional 

 scale in connexion with ef Rock -jointing in its relation to Phe- 

 nomena in Physical Geography and Physical Geology/' and as 

 the subjects it treats of are of general importance, and exciting 

 much attention just now, I may be excused introducing them 

 into the present note. 



In my Report on Jointing and Slaty Cleavage, which appears 

 in the ' Transactions of the Royal Irish Academy/ vol. xxv. 

 (1875), reasons are given in favour of the doctrine that jointing 

 is a physical phenomenon, which, constituting lines or zones of 

 weakness in the earth's crust, has permitted subterranean dis- 

 turbances, often accompanied by igneous upbursts, to follow the 

 courses of these zones*. 



With respect to slaty cleavage, in consideration of various 

 evidences adduced in the Report, I have for several years past 

 maintained that it is the result of pressure consequent on 

 subterranean disturbances exerted against planes of jointing 

 (Bangor slates), or depositional partings (Delabole slates), 

 thus bringing them into approximate or immediate contact. 



Since the Report appeared, Daubree has given a description 

 of some experiments which he regards as proving the mechanical 

 origin of jointing. By means of torsional pressure applied to 

 plates of ice, he has developed, in the latter, groups of " approx- 

 imately parallel" lines of fracture, crossed by other lines 

 nearly at right angles. But I cannot regard these experiments 

 otherwise than as simply illustrating- the well-known truth that 

 similar effects are produced by dissimilar causes ; and it may be 

 strongly contested that ice, crystalline in its origin in which it is 

 doubtful that structural planes, original or superinduced, are ever 

 absent is a suitable substance on which to experiment by way 



* This doctrine suggested itself to me on my becoming acquainted with 

 an early speculation in dynamical geology of Phillips's. See op. cit. pp. 637 

 and 638, and Phillips's Report Brit. Assoc. 1834, p. 657. 



