HEAT VEINS. 



337 



The process decomposes the walls of the fissures or cavities to make 

 the lilliug materials, the walls showing it by their decayed condition. 

 The lateral source may be within an inch or a few inches of the place of 

 -deposition ; and still it well illustrates much of vein-making. Bitumen or 

 mineral oil may also be taken in from carbonaceous shales, and deposited in 

 the amygdaloidal cavities and fissures ; and to its presence J. Lawrence 

 Smith attributed the reduction of the magnetite in igneous rocks to 

 grains of native iron, and even the production of the great masses of native 

 iron brought to the surface by basaltic ejections in Greenland. 



The term vesicle, as applied to a vapor-blown cavity in an igneous rock, 

 has been put into Greek form in the word litliophysa (stone-bubble) by 

 Kichthofen (1860), and applied especially to peculiar chambered cavities 

 common in obsidian, its variety, lithoidyte, and in rhyolyte. They occur 

 in great perfection, as flattened spheroids, in the region of the Obsidian 

 Cliff, Yellowstone Park (Fig. 279, page 306). The following figures are from 



310. 



311. 



512. 



313. 





Lithophysae of the Obsidian Cliff. Iddings. 



a memoir by J. P. Iddings (1888). Three of the lithophysae are shown, 

 of natural size, in Pig. 310, and three others in section in Pigs. 311, 312, 313. 

 The rock containing the lithopliysae commonly consists of alternating solid 



Dana's manitai, — 22 



