3i6 



PRINCIPLES OF STRATIGRAPHY 



led the observers to the conclusion that such features are reliable 

 as indications of surface flows, yet there are cases in which these 

 surfaces have such an intimate relation with marine sediments as 

 to suggest the possibility of intrusion. An example of this kind is 

 described by Fox and Teall {12:211) from the Greenstone of the 

 Lizard and Mullion Island, where intimate association with radio- 

 larian cherts suggested that the lava was intruded between the 

 sheets of chert near the surface of the sea bed upon which they 

 were being deposited. This intimate association with radiolarian 

 cherts also found in the Arenig lavas of Great Britain seems at 



Fig. 56. Spheroidal or pillow lava with variolitic selvages. North end of 

 Le Chenaillet Ridge, above the Durance Mont Genevre region, 

 France. (After Cole and Gregory.) 



present the only good indication of the probable submarine origin 

 of the lava. So far as the pahoehoe type of surface is concerned, 

 it appears to be equally characteristic of subaqueous and subaerial 

 extravasations. That constant and reliable minor differences exist 

 between subaerial and subaqueous lava surfaces is scarcely to be 

 doubted, but at present such differences appear to be unrecognized. 

 The aa type of lava has also been recognized in older formations. 

 In the pre-Cambric rocks of the Vermilion iron-bearing district of 

 jNlinnesota occur bunches of igneous rocks having a concentric 



