392 EHREXFELD— JOINTING AS A FUNDAMENTAL FACTOR 



sediments and in other cases as on Tenerif, Hierros and Antaos, 

 the incoherent volcanics carry loose fragments of " older or at 

 least coarsely crystalline rocks " which Gagel considers may have 

 come from an older deep-seated "socket" or portion of the islands. 



There is presented here then a construction in which may be 

 seen two phases of joint influence, the broken and more or less 

 symmetrically arranged fragments of residues of a continental struc- 

 ture, and secondly the rise of volcanic masses to the surface through 

 the open fissures which lie between the aforementioned segments. 



The Azores show likewise recent eruptive products with ap- 

 parently no sediments earlier than middle Miocene ; there is the 

 further fact of deep sinking of the sea floor between some of these 

 islands, as over 6,000 feet between Graciosa and Corvo.*^ 



The general position of all these island groups of the Atlantic, 

 their relations to the surrounding lithosphere shapes are all of im- 

 portance ; the positions of the Canary-Madeira group near the 

 great adjustments incident to Mediterranean conditions, and the 

 position of the Azores on the great Atlantic plateau are part of the 

 general idea that these are all structures which are to be regarded 

 as .behaving in the same way as similar structures do on those por- 

 tions of the lithosphere which are now continental. There are 

 here evidences of the controlling action of a system of joints which 

 are believed to have played their part not in surface degradation in 

 the usual sense, but have been active in a system of movable seg- 

 ments which wdien under stress have resulted most probably in an 

 essential up and down movement. 



I have attempted to demonstrate block or joint development in 

 , the first place as an integral part of the lithosphere itself in a man- 

 ner analogous to its presence shown in the larger land masses. 

 That it has been the controlling factor in the disintegration of a 

 former land mass represented by the islands now under discussion 

 is too much to take as anything which can so easily be demonstrated. 

 That it has been an active factor in marine denudation at this locus 

 seems to me rather more than a supposition and appears as prob- 

 able from the action of joint planes to produce an acceleration of 

 marine degradation elsewhere. If these ideas be carried to the island 



*i Gagel, op. cit., p. 9. 



