594 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



pillow greenstones of Alaska (plate 19) and in the modern lavas of 

 Hawaii and Samoa (plates 22 and 23). 



The individual pillows are very commonly sheathed in a film of glass 

 from 2 or 3 to 25 or more millimeters in thickness (plate 16), and the 

 microscope shows that this passes gradually into the crystalline lava of 

 the interior. In many cases the rock is massive or only slightly vesicular, 

 although some very light and spongy lavas are also known in this form. 

 A vesicular or variolitic zone (or both) very commonly lies just within 

 the glassy crust, and in some localities a flow-structure is developed 

 parallel to the outer surface of each mass (plate 20, figure 1 ; plate 21, 

 figures 1 and 2 ) . Two or more concentric layers of alternating vesicular 

 and solid lava are sometimes found, and the central portions of some 

 pillows are extremely porous or even cavernous, the weathering out of 

 such spongy interior leaving only hollow shells (plate 20, figure 2). 

 Some nonvesicular spheroidal lavas also have in many of the masses a 

 central or axial hollow resembling the pipe in a steel ingot (plates 16 and 

 IT, figure 1). 



The vesicular portions of pillow lava have been quite generally con- 

 verted into amygdaloid, and cavities of all kinds are commonly more or 

 less completely filled with the various minerals (calcite, zeolites, etcetera) 

 previoush' referred to. In such cases the occurrence of empty spaces, 

 either vesicular or interspheroidal cavities in the outcrops of the rock, is 

 the result of the removal of these minerals by weathering. 



A pronounced radial columnar jointing is very commonly developed in 

 the pillows, and when these are broken down by the mechanical processes 

 of weathering or the corrasive action of waves and streams they fall into 

 tapering, pointed or wedge-shaped fragments (plates 15, 17, figure 1, and 

 plate 19, figure 2). A concentric jointing or lamellar structure, so char- 

 acteristic of spheroidal weathering, has been observed far less commonly 

 (compare plate 21, figure 2, and plate 25). 



In the metamorphic types of lava the characteristic structures are more 

 or less masked by the results of subsequent processes. One of the most 

 common, which is quite characteristic of the greenstones, is the develop- 

 ment of chlorite from the glassy sheaths of the pillows and from the frag- 

 ments in the interspaces, accompanied by the partial or complete chloriti- 

 zation of the ferromagnesian minerals throughout the mass of the rock 

 (plate IT, figure 2). The chlorite in the interspaces generally has a 

 schistlike structure parallel to the surfaces of the adjacent ellipsoids. 

 This fact led some of the earlier observers to ascribe the pillow structure 

 altogether to earth movements in which the lava had become involved, 

 producing a gigantic brecciation and rubbing of the blocks on each other. 



