56 CLASSIFICATION IN LITHOLOGY. — GENERAL CONCLUSIONS. 



When complete (bauscli) analyses are made of typical rocks, rock-species 

 are believed to have in their broader features certain limits of chemical 

 composition outside of which the normal forms rarely go, and inside of 

 which the normal forms of other species rarely come ; but the mineralogical 

 composition is more or less unstable and variable, depending upon alteration 

 and other conditions to which the rock has been subjected. It is thought 

 that the chemical relation of rock species would be much better shown if 

 the percentages were expressed in terms of the elements, instead of in their 

 compounds — as Nordenskiiild has suggested for the meteorites. 



All I'ocks, except meteoric and recent volcanic ones appear to be more or 

 less altered ; and it is evidently from these altered rocks that the classifica- 

 tions and principles of classification have been chiefly derived in Europe, on 

 the supposition that as these rocks are now found to be, so they always were, 

 and always will be. 



Fragmental or derived rocks ought to be classed, so far as it is possible, 

 under the rocks from which thoy were derived ; the object of the classifica- 

 tion both of these and the massive rocks beiufr to show their relations and 

 derivation so for as practicable. 



The relation of one rock to its fellows in the field is the principal crite- 

 rion for determining its origin — particularly in the case of altered rocks. 



Taking the consolidation of any rock as the datum point, the minerals 

 and rock fragments are found to be naturally divided into three distinct 

 classes of different origin : — 



1. Minerals and fragments of prior origin. 



(«) Those characteristic of the rock species. 

 (l/) Those that are accidental. 



2. The jiroducts of the consolidation. 



3. The products of alteration and infiltration. 



All may exist together in a rock, or all except those of one class may be 

 wanting ; and far more depends, in lithology, on being able to determine the 

 origin and relations of these different classes of minerals than on ability 

 to name the mineral species correctly. Minerals of the third class are 

 apparently formed through the agency of percolating waters, which may 

 be hot or cold. 



The alterations appear in general to take place slowl}^ ; while rapid alter- 

 ations, such as would be produced by hot, intensely active waters, generally, 

 if not always, seem to bear distinctive marks. 



