Il8 THE MICROSCOPE. 



combined, and at the same time affords valuable in- 

 formation as to the physical structure and arrangement 

 of the components of the rock mass, tending to 

 elucidate its formation and origin." Let me select 

 one or two instances out of several given by Mr. 

 D. Forbes. You are, perhaps, acquainted with the 

 mineral termed Obsidian, or Volcanic glass, which is 

 produced by the fusion of felspathic rocks or those 

 which contain alkaline silicates. The glassy appear- 

 ance testifying to an apparently complete vitreous 

 condition would, at first sight, defy all attempts to 

 discover the structure ; nevertheless, some part of the 

 mass will be found to be sufficiently devitrified to 

 allow of its structure and mineral composition being 

 recognised, and Mr. D. Forbes has figured a very 

 pretty section of obsidian in which the pyroscenic and 

 felspathic constituents of the rock are very clearly 

 apparent. Rocks, according to their structure, fall 

 naturally into one or other of two great classes (i) 

 Primary, or Eruptive; and (2) Secondary, or Sedi- 

 mentary. Now, in some cases it is impossible to 

 determine by mere ocular inspection to which of 

 these classes a certain rock may belong. Microscopic 

 examination shows that whatever be the geological 

 age of these primary rocks, or from whatever part of 

 the earth's surface they may be taken, they " possess 

 certain general and definite structural characters dis- 

 tinguishing them at once from all other rocks." 



There occurs, either found embedded in or breaking 

 through the coal-measures of Staffordshire, a rock 

 popularly termed " White Horse," from often having 

 the appearance of a whitish clay ; the coal-measures 

 at points of contact with the rock are frequently burnt 

 and altered. " The origin of this rock, whether sedi- 

 mentary or igneous, was disputed until the more 

 recent geological and chemical examinations of it 



