394 EHRENFELD— JOINTING AS A FUNDAMENTAL FACTOR 



in the more careful study of these islands as developments from 

 jointing or segmented structures. 



The marked character of volcanic products from the island vol- 

 canoes, such as the successions of andesitic lavas supposed to be 

 characteristic of the Pacific type compared for example with those 

 from the Philippines which are much more varied though frequently 

 andesitic, and with those from the Sandwich group which are essen- 

 tially basic, may perhaps show some solution to the physiographic 

 development of the region indicated. 



That the igneous history of some of these groups has been a 

 very long one is indicated by the same sort of reasoning as applied 

 to rock masses elsewhere. This may be taken to mean a long period 

 of time in which the segmented condition or open fissure condition 

 has prevailed. 



It is not accidental nor casual but from the evidence at hand we 

 may with reason conclude that the surface of the lithosphere be- 

 neath the hydrosphere in this Pacific district of Oceania has been 

 subject to tectonic movements, fissuring extrusion of igneous masses 

 in a manner analogous to other parts of the lithosphere such as 

 continental masses in which a jointing or block structure may be 

 shown to be present. 



Connected with this discussion is the general structure of the 

 solid portion of the earth, now commonly referred to as the zones 

 of fracture and flowage. These, as is known, are divided into 

 more or less definite lines so that under certain supposed pressures 

 rock masses will move in flow lines rather than in fracture lines, 

 such as is indicated in many diagrams of mountain making. This 

 may be regarded by some authors as a factor against the proposi- 

 tion of joint control which I have proposed. Further reflection, 

 however, will suggest the thought that while the zones of fracture 

 and flowage do represent definite differences in the behavior of the 

 rock masses of the earth, that these lines of zone are not, however, 

 fixed in their actual distance below the surface of the lithosphere. 

 In other words, it is not only possible to conceive but I think the 

 circumstances compel us to conceive of a certain rock mass passing 

 from the flowage zone to the fracture zone by release of pressure. 

 That is, if we take certain types of mountain structure represented 



