266 DYNAMICAL GEOLOGY. 



The common effects on the rocks are : the covering of small surface spots, 

 or depressions, with beads of glass, or with sinuous glassy lines sometimes 

 radiating; the production of tubes sometimes half an inch or more in 

 diameter, descending with diminishing size a few inches into the rock, and 

 sometimes dividing downward ; the glass being such as the rock, or Some of 

 its more fusible ingredients, would afford on fusion. The usual absence of 

 microlites is regarded as an indication of the sudden cooling of the glass. 



Specimens from Mount Tliielson, Oregon, south of the Columbia, where the rock is 

 hypersthene-basalt, consist, according to Diller, of a coating of patclies and beads of 

 crlass, and also of tubes \ to f inch in diameter, having a brownish glass within, which 

 descend 2 to 3 inches into the rock. On West Peak, east of the Sangre de Cristo Range, 

 Col., tubes glassy within, and surface-depressions with beads of glass, occur in augite- 

 dioryte, and in one place a tube appeared to follow the course of a small vein of ore 

 (R. C. Hills, 1890). A fulgurite-glass, occurring in the Alps on Mount Viso, coating fur- 

 rows made in glaucophane schist, was peculiar in containing microlites (Rutley, 1889). 



The disrupting power of lightning is sometimes shown in the fracturing of rocks, and 

 it is supposed that this may have been, in past time, an important agent of rock-destruc- 

 tion. But this theory is opposed by the fact that the strokes producing fulgurites have 

 done very little shattering. 



225. 



Vesuvius as seen from the harbor of Naples. D. July, '34. 



The burning of coal-beds has produced scoria and other igneous results in North Dakota 

 and Montana. But the mode of ignition of the beds is not known. A stroke of lightning 

 is the most probable agent. It is hardly possible that chemical changes ever occasioned 

 it. In the States above mentioned the burning of coal-beds of the Lignitic Tertiary has 

 changed clays to hard and sometimes porcelain-like rocks, usually reddening them, and 

 also to beds of a half-fused cellular or scoriaceous and pumice-like character, looking like 

 the products of a volcano. One of the regions thus burnt over, on the Little Missouri, 

 is 20 to 30 miles broad by 200 miles in length. Others occur in the Yellowstone at the 

 mouth of Powder River and along the latter stream ; about the sources of Tongue River, 

 within a few miles of the Big Horn Mountains, and on the north fork of the Cheyenne 

 River, as observed by Hayden. Fragments of pumice have been found on the Missouri 

 as far south as Pierre, and the early explorers supposed them to be the products of 



