CHARACTERS OF PILLOW STRUCTURE 595 



As already suggested, other early attempts to account for the structure 

 include hypotheses of spheroidal jointing due to contraction while cool- 

 ing, exfoliation due to weathering, and even concretionary processes. 

 Among more recent investigators some have likened the rounded forms to 

 the ropy flow structure of a viscous pahoehoe; others compare them to 

 block lavas of the rugged aa type, assuming a certain degree of residual 

 plasticity in the angular blocks. Subaerial flow, specified or implied, is a 

 necessary condition in some hypotheses; but probably the majority of 

 recent students of this phenomenon ascribe it to the influence of water 

 on subaqueous eruptions or to the flow of lava into bodies of water or 

 over water-bearing silts. Various combinations and modifications of 

 these hypotheses have been advocated by different investigators, and some 

 geologists hold that the evidence establishes the fact that pillow structure 

 may be produced under a wide range of conditions. 



Every known example of this structure occurs in basalt or some closely 

 related variety of basic igneous rock. The greenstones in which it is so 

 abundantly developed in the Lake Superior region and at many localities 

 in Europe are no exception to this rule, since they are clearly recognized 

 as chiefly basaltic rocks that have been modified in varying degree by 

 hydration and mechanical processes. The peculiar basic rocks, with 

 "albitized^' feldspar and '^^chloritized" pyroxene, to which the name spilite 

 has been given, are doubtless also modified basalts. In one or two exam- 

 ples the rock possessing this structure has been called a basic pyroxene 

 andesite. 



Distribution of Pilloav Lavas 

 in general 



The wide range of pillow lavas, both geographically and geologically, 

 their characteristics in diflerent localities, and incidentally the persistent 

 confusion of theory concerning the origin of the structure may best be 

 indicated by a brief review of the descriptions of the better known occur- 

 rences ill various countries. T have included here references to all litera- 

 ture on the subject that I have been able to find in a rather limited search 

 of the larger scientific libraries of 'New York and Washington. The list 

 is known to be incomplete, however, at least for Germany and the British 

 Isles, and is probably so for other countries as well. 



GERMANY 



Saxonij. — As early as 1(S34 C..F. T^aumanii* described the conglomerate- 

 like appearance of the greenstone schists of Hainichen, in the Kingdom 



* Erlauterungen zur geognotische Karte des Konlgreichs Sachson, pt. 1, 1884, p. 69. 



