CHAHACTERS OF PILLOW STRUCTUKE 593 



grained than the inside, and has rows of small vesicles running parallel to 

 the outer surface. In the interstices between the blocks various sedimentary 

 materials have sometimes been introduced, such as volcanic tuff, sandstone, 

 shale, ironstone or chert." 



The forms described are separate or nearly separate masses of lava that 

 yield rounded or oval cross-sections in all directions, tiiough in many 

 localities they are molded in varying degree on one another or even flat- 

 tened out like cushions or half -filled sacks, and in the latter case nearly 

 or quite fill the space, without open interspaces (plates 15-21).''^ These 

 forms are to be clearly differentiated from the spheroidal jointing or ex- 

 foliation due to weathering (see plate 25). Where spaces exist between 

 the masses they tend toward a rude tetrahedral form, with concave sides 

 (hence are roughly triangular in cross-section), and are generally filled in 

 part with flakes and angular fragments of the same character as the 

 curved surfaces of the hounding masses. Filling the remaining spaces in 

 many localities and cementing the fragments into a breccia are a great 

 variety of minerals that are commonly considered secondary. Prominent 

 among these are chlorite, calcite, quartz, chalcedony, agate, and other 

 cherty and flinty forms of silica, together with epidote and the wonder- 

 ful series of the zeolites. In some regions but few of these minerals 

 occur, while elsewhere, even in other parts of the same flow, they are de- 

 veloped in marvelous variety and abundance. 



In some districts the interspaces are filled with radiolarian and other 

 cherts, jaspers, and limestones or with shales and coarser terrigenous 

 sediments. Some of these seem to have been gathered up from preexist- 

 ing oozes or muds across or into which the lava flowed, and others appear 

 to have fallen into the open spaces from subsequent deposits on the lava 

 surface. 



In many places the rounded masses are elongated and more or less 

 flattened into a bale-like or bolster-like forms, and indeed some degree of 

 elongation is a very general characteristic (plate 18, figure 1 ; plate 21, 

 figure 1 ) . In such cases there is commonly also a pronounced parallelism 

 of the longest axes of the masses and a like, though less prominent, paral- 

 lelism of the intermediate and shortest axes as well. Gradations into 

 more irregular twisted and ropy lavas have also been observed, with half- 

 formed pillows or ellipsoids attached to each other and to the solid massive 

 lava of the fiov/ by necks or along the sides, as in the typical Mesozoic 



3 Hence Geikie's term, pillow lava, has been adopted here because it seemed, on the 

 whole, more accurate and more appropriate for general application than such terms as 

 spheroidal, globular, and ellipsoidal. Tillows, sacks, and cushions are subject to a wide 

 range of form and the comparison suggests a truer conception of the structure than any 

 of the other terms. 



