MINOR STRUCTURES OF VOLCANTCS 317 



structure. They have been referred to the pseudo bombs of the 

 aa. (Clements-2.) Some of the Triassic extrusives near Green- 

 field, Massachusetts, have been referred to this type by Hitchcock. 

 (16:283.) 



2. Acid lavas. These are as a rule very viscous and slow- 

 moving, and may solidify before they spread far. The surface is 

 generally rough or ropy, the former being a feature of some acid 

 lava flows of Vulcano in the Lipari group (obsidian), the other 

 being illustrated by lavas of Vesuvius. As shown in the Vesuvian 

 stream of 1858, which was very viscous and slow moving, the sur- 

 face has been wrinkled and folded in quite a remarkable manner, 

 some of the folds closely resembling coils of rope. This is the sur- 

 face feature seen in artificial slags, flowing from a furnace. The 

 cause of this appears to be the wrinkling of the chilled surface 

 crust through the continued onward movement of the liquid mass 

 below. Sometimes the breaking of the lava crust produces a heap 

 of large and small fragments or blocks, so that the lava stream 

 looks like a huge mass of broken fragments confusedly piled to- 

 gether. (Block lava, Schollcnlava.) Sometimes lavas of the more 

 acid type are very liquid and flow rapidly. In such cases a rough 

 and ragged cindery surface is produced suggestive of aa lava. The 

 surface suggests the solidification of a boiling, squirting mass 

 (sprafsige Lava). Such a surface is seen in the lava stream of 

 1872 on Vesuvius. In rapidly cooling lavas the surface of the 

 stream may be covered with a crust of hard slaggy material which, 

 breaking and rolling under at the front of the moving lava strearn, 

 forms a slaggy floor on which the more compact lava comes to rest. 

 This crust of slag is responsible for the relatively little effect which 

 the lava of a submarine volcano produces on coming in contact with 

 the water, or for the phenomenon of a lava stream flowing across a 

 snowfield without completely melting it. (Credner-4 i/jo.) 



Minor Structural Characters of Volcanic Rocks. 



Flo7v Structure. A banding of igneous rocks is often noted, this 

 banding sometimes simulating stratification, for which it has at 

 various times been mistaken. It is produced by the disposition of 

 the crystals, vesicles or other recognizable structures in more or 

 less parallel lines, which, however, are constantly interrupted by 

 obstacles around which the lines curve in such a manner as to show 

 that it was due to the flowing of the viscous mass around the ob- 

 stacle. Flow structure is not confined to surface flows, but also 



