9E Outlines of Geology. 



chemist. A knowledge of the crystalline forms, and general 

 mechanical characters of substances must often be called to the 

 aid of his speculations, and he can frame no plausible theory 

 without combining with such information a just, and even minute, 

 acquaintance with the effects of heat and various solvents upon 

 mineral masses, and upon the parts of which they consist. 



Without these guides, therefore, he who aims at any thing 

 beyond mere practical geology, will infallibly go astray : they 

 are lights essential to his successful progress, and the wanderings 

 of those who would penetrate into the more secret parts 

 of geology without them, are abundantly manifest to the intel- 

 ligent reader, in most of those speculative writers, whose 

 eloquence and dexterity in argument may mislead the unwai'y 

 into an acquiescence in their reasonings, but which, when 

 measured by less falliable standards, are found void of solidity 

 and truth. 



Siliceous, calcareous, and argillaceous substances, either pure 

 or nearly so, and in a state of mixture, or loosely and indefinitely 

 blended, rather than in strict chemical combination, constitute a 

 very large relative proportion of those rocky masses, or scattered 

 or comminuted substances, which form, or have formed, the most 

 exterior constituents of our planet ; and of these, considered in the 

 abstract, the chemical and mineralogical history is soon told. 



Under the name oirock crystal, or quartz, silica presents itself 

 nearly pure ; and in the varieties of flint, agate, calcedony, and sili- 

 ceous sand, it is, by far, the predominating ingredient. Its most 

 usual crystalline form is a six-sided prism, terminated by a six- 

 sided pyramid ; but its primitive form is an obtuse rhomboid, 

 nearly approaching the cube. In specific gravity the pure 

 varieties fluctuate a little on one side or other of 2.6. It cleaves 

 with great difficulty ; its common fracture is conchoidal. There 

 are many varieties as to form and colour, which chemical mine- 

 ralogists describe and distinguish ; they are all. hard enough to 

 scratch glass. The leading chemical characters of silica are, 

 extreme difficult fusibility; insolubility in water, and in nearly 

 »M acids, except under certain peculiar circumstances of recent 



