VOLCANOES. 741 



rocks, doleryte and peridotyte, and the lighter feldspathic or acidic 

 kinds, felsyte and others allied, are the most common. They are 

 sometimes cellular, owing to inflations by steam or other vapors ; but 

 the cellules have generally a smooth or even surface within, and are 

 not ragged like those of lavas, — a fact due to their having been under 

 pressure when formed. Such cellular kinds have the cellules filled 

 with minerals of subsequent origin, and thus become the rock called 

 amygdaloid (p. 66). The manner of filling these cavities, and the na- 

 ture of the materials, are explained on page 777. Amygdaloidal varie- 

 ties of dolerytic rocks usually contain considerable moisture, and often 

 also disseminated chlorite. They thus show that they were subjected to 

 a free supply of moisture, from a subterranean source, when in process 

 of eruption ; and this fact accounts for the existence of the cellules. 



These igneous rocks sometimes form layers, interstratified with ordi- 

 nary sandstones or other sedimentary rock, and even uncompacted sand 

 and gravel ; showing that they having flowed out over a region.it may 

 be for hundreds of miles, covering up the strata previously laid down, 

 and then becoming the basis for new deposits of sand oy mud. They 

 thus lie between beds in all the geological formations. Examples of 

 American Lower Silurian igneous rocks of the kind are described on 

 page 185. The Triassic or Jurassic trap, on the Atlantic border of 

 North America, affords another example, as described on page 418 ; but 

 the beds here have come up through sandstone rocks, without exten- 

 sive overflows. The Cretaceous era, and still more the Tertiary and 

 Quaternary, were remarkable for the extent of the eruptions over the 

 western slope of the Rocky Mountains (p. 524), and also in Britain 

 (p. 525) and many parts of Europe, and on other continents. The 



Fig. 1120. 



Sections of Table Mountain : A, at Maine Boys' tunnel ; B, at Buckeye tunnel. 



lava floods that spread over the western slope of the Sierra Nevada 

 (p. 797), were so vast that, as Whitney states (Geological Report, 

 1865, and Auriferous Gravels, 1879), many river valleys were oblit- 

 erated, and the rivers were forced to begin erosion anew along other 

 lines. Although having only the Quaternary to work in, the moun- 

 tain streams have cut up large parts of the lava-covered region into 

 "table mountains," and dug their new channels, to a depth, in many 

 cases, of 1,000 to 3,000 feet. The preceding figures, from Whitney, 



