xJ\ XxX. XJ- 



THE KINDS OF ROOKS 



^ ' Some rin up hill and down dale knapping the chucky stones to pieces wi 

 hammers like sae many road-makers run daft. They say it is to see how the 

 warld was made.'' — St. Monan's Well. 



Reference has already been made to the fact that but 

 sixteen out of all the known elements enter into the compo- 

 sition of the earth's crust in other than comparatively minute 

 quantities. Also to the equally important fact that the com- 

 bination of these elements as represented in not above a score 

 of well-known mineral species go to make up the essential por- 

 tion of nearly all rock masses. Nevertheless, owing to the 

 variety of forms under which these rock masses occur, the vary- 

 ing conditions under which they originated, or the proportional 

 quantities of the various minerals which they may contain, we 

 find numerous and widely varying types of rocks, a satisfactory 

 consideration of which necessitates first some attempt at syste- 

 matic classification. It may be said at the outset, however, that 

 rock species, in the sense in which the word is used in mineralogy 

 and zoology, scarcely exist. It is true we may have, and par- 

 ticularly among igneous rocks, certain forms which on casual 

 inspection, or indeed on close inspection, with regard only to 

 limited geographical areas, seem to possess an individuality of 

 their own sufficient to entitle them to being considered as true 

 species. Yet, when we come to compare these with others, to 

 take into account their physical and chemical composition, their 

 structure and mode of occurrence, and above all to consider how 

 any rock varies within its own mass, and the still greater varia- 

 tion which may have been produced through alteration, it will 

 be seen that one form grades into another almost without limit, 

 that, indeed, no two are exactly alike, and that, were we to 

 attempt any hard and sharp lines of discrimination, our species- 

 making would practically resolve itself into an enumeration of 

 individual occurrences. This fact will become apparent as we 



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