596 VOLCANIC ROCKS. 



In the same place they are often repeated in sim- 

 ple alternations, having originated in successive 

 eruptions ; or else they are found forming alter- 

 nations of solid rock with various loose matters. 



These loose materials, which constitute a 

 principal division of the volcanic substances, 

 consist of various rocks, more or less altered by 

 fire, and often reduced to powder, in which case 

 they have improperly been termed ashes. Be- 

 sides these, volcanoes are known, in certain cases, 

 to eject water ; and the deposits then form mud, 

 which, on drying, is consolidated into a tufaceous 

 or clayey mass. 



Respecting the modes of structure in these 

 rocks, it is unnecessary to enter into minute de- 

 tails ; as it would be merely to repeat much of 

 what has already been said in treating of the trap 

 family. Of the larger kinds, the prismatic and 

 the lamellar both occur ; and, of the smaller, the 

 cavernous, amygdaloidal, and porphyritic, are 

 very predominant. Such details as are more par- 

 ticularly necessary will be found in the Synopsis. 



Of the mineral composition, there is equally 

 little to be said. In all the fundamental points, 



