180 SCARBA. GEOLOGY. 



that they should occur in these islands.* It may for ever 

 remain impossible to explain the causes by which even 

 their present distinctness has been produced, and their 

 boundaries determined. It can only be conjectured, that 

 the differences in the constitution of the rocks from the 

 ruins of which they have been formed, those of the cavi- 

 ties in which they have been deposited, or of the tides 

 and currents to which they have been subjected during 

 their deposition, have, under various modes of combina- 

 tion, produced all the variety of appearances now visible. 

 I need scarcely point out, that in this case also, the alter- 

 nations of fine and of coarse argillaceous schist (slate 

 clay) with the secondaiy sandstones, present a striking 

 resemblance to those of the primary strata under review. 

 From these and other instances occurring in the course 

 of this work, it may safely be concluded, that the present 

 theory of a certain fixed order of succession in the pri- 

 mary rocks is still imperfect; since every day produces 

 fresh reason to doubt our knowledge of its nature and 

 limits. 



In dismissing this subject it is necessary to make some 

 remarks on the term graywacke, since it has here been 

 used in a sense more extended than that which is generally 

 received. It has been enumerated only among the rocks 

 called transition, while it has here a place among the 

 primary. There can be no dispute respecting the defini- 

 tion, which is abundantly clear; nor can any essential 

 difference be found among the several varieties of it 

 which occur in the situations now described, and those 

 found on the confines of the secondary strata, to which 

 the term transition has been applied. Particular speci- 



* That observation may be more generally extended to all the mixed 

 rocks. Among the infinite possibilities of mixture, they are limited to 

 a very few; and their characters, under modifications which do not 

 affect this general conclusion, are surprisingly constant. It is not 

 the least remarkable part of this law, that the same mixtures arc 

 found without material variations of character over the whole globe. 



