XXXll INTRODUCTION. 









istics derived from their exterior forms ; while, in mineralogy, 

 the forms of the greater masses being casual and uncertain, 

 recourse must be had to chemical analysis, as well as to exte- 

 rior qualities. But, in the former, it is not so much the 

 substances forming the combination, as the mode or manner 

 of that combination, which constitutes the essential differ- 

 ence among the objects of mineralogical knowledge 5 for the 

 diamond is only a modification of coal, and the sapphire of 

 clay and iron rust. Hence, while the mode of the chemical 

 combination establishes the most essential difference, the 

 structure and the aspect constitute more minute distinctions. 

 It may also be observed, that Werner's method of distin- 

 guishing rocks, by their formations and positions, seems at 

 variance with his treatise on the external characters of mi- 

 nerals, in which the science is rightly founded on its only 

 firm foundations, those depending on the characteristics of 

 the substances, as considered in themselves. This object was 

 perhaps considered by Werner as already accomplished in that 

 treatise, and in his lectures on Oryctognosy, or the general 

 knowledge of minerals : but the rocks form a class so im- 

 portant and distinct, that they deserved a separate considera- 

 tion, before proceeding to the bold design of general geo- 

 gnosy or geology. Perhaps the experience and observations 

 of two centuries may still be wanted, before such a design 

 can be reasonably attempted ; and at present the Huttonian 

 system has as many admirers as the Wernerian, though 

 founded on principles totally opposite : but, in all events, it 

 was necessary to begin with an elementary work, containing 

 all the erudition already acquired on the subject, leading to 

 more clear and precise views, or exact distinctions, and a 

 Necessity of consequent increase of denominations, without which even 

 must be embarrassed ; for at present it is not even 

 agreed what object precisely constitutes granite, and what 

 object basalt. All theories, however, tend to the advance- 

 ment of science, by stimulating inquiry and discussion ; but 

 it is clear that the theories must be vague, and the contests 

 .alike fruitless and endless, till the parties shall have agreed 



