FEATURES OF LAVA FLO\\"S 315 



or less rounded and boulder-like, and vary in size from an inch or 

 less to ten feet or more." (Dana-8: 10.) 



Peach and Home have described and figured the lava surfaces 

 of the Ordovicic rocks of the south of Scotland. (21.) One of 

 their striking characteristics is the pillow-shaped or sack-like form 

 which they present. "On a weathered face they sometimes look 

 like a pile of partially filled sacks heaped on each other, the promi- 

 nences of one projecting into corresponding hollows of the other." 

 (Geikie-13 :/p5.) The rocks are finely amygdaloidal, the vesicles 

 being grouped in lines parallel to the outer surface of the pillow-like 

 block in which they occur. 



The spaces between the subspheroidal masses are sometimes 

 filled with fine sediment, sometimes with fossiliferous limestone, 

 and again with radiolarian chert, these being deposited upon the 

 surface flow after cooling or sometimes before. Some of the radio- 

 larian cherts, how^ever, appear to be deposited contemporaneously 

 with the lava, for they are intercalated with it. These lavas are 

 believed to represent submarine eruptions, at successive periods, 

 while between these periods, normal sedimentation took place. 

 Structures of this kind have been found in the lower Ordovicic 

 (Arenig) lavas of Cader Idris, IMerionethshire, North England. 



A similar structure occurs in the post-Carbonic (Tertiary) vari- 

 olitic diabase of the Mont Genevre district in the French-Italian 

 border. (Cole and Gregory-3.) The structure of this lava is 

 spheroidal on a large scale, most commonly "resembling pillows or 

 soft cushions pressed upon and against one another." As shown in 

 the cliffs, they appear as swelling surfaces with curving lines of 

 junction. (Fig. 56.) Small vesicles occur in the rude spheroids, 

 especially toward the margin, while in some places the whole rock 

 becomes vesicular and slaggy. The surfaces of these masses are 

 covered by a crust of variolite, from i to 7 or 8 centimeters thick, 

 the variolites being grouped or drawn out in bands parallel to the 

 surface and varying from almost microscopic to a diameter of 

 5 cm. 



Ransome finds a structure of this character in the basic lava of 

 Point Bonita. Marin County, California, and comes to the conclu- 

 sion that it is essentially a flow structure and that hence the rock in 

 question is an extravasated lava. {27^.) He regards the structure 

 as indicating a lava of intermediate viscosity between that pro- 

 ducing the pahoehoe and that resulting in the aa surface. 



Crosby has described structures of this type from the Carbonic 

 lavas of the Nantasket region of eastern Massachusetts. (5.) 

 While the careful study of these structures in the cases cited has 



