456 BUTE. GEOLOGY. 



it frequently passes into chlorite schist, or at least ap- 

 proximates to that substance ; the mica acquiring a 

 greenish hue and a more tender texture. This change 

 is most remarkable on the western extremity of the boun- 

 dary ; and it will also be found to predominate through 

 Argyllshire, and even to a greater distance, in the north- 

 easterly direction, until it ultimately ceases. 



The characters of the external belt of primary strata 

 have been cursorily mentioned in treating of Arran, but 

 they will require a more detailed description. That is 

 rendered peculiarly necessary, not only by their import- 

 ance, but by their unexpected nature; if that may be 

 considered unexpected where the opposite expectations 

 have arisen from systematical views or from imperfect 

 observation. It will be convenient to distinguish this 

 by the title of the argillaceous series ; as the presence 

 of argillaceous schist forms one of its most characteristic 

 features. 



The substances which occur in this series are the 

 following; micaceous schist, chlorite schist, clay slate, 

 and graywacke ; each of them presenting variations that 

 will require some additional details : to these may also 

 be added limestone, although this appears to be rather 

 an incidental than a necessary member. 



The micaceous schist presents many of its more usual 

 varieties, but it occurs also under a form in which, as far as 

 I know, it is not found in any other place than on the 

 confines of the argillaceous series. This variety is charac- 

 terized by the distinctness of the quartz grains in its com- 

 position ; consisting, in fact, of an aggregate of sandy or 

 gravelly particles united by scaly mica; and resembling 

 in texture some varieties of graywacke. The grains 

 vary in size from that of a pin's head to a quarter of 

 an inch in diameter; but the several sizes are never 

 intermixed in the same place; each distinct portion 

 presenting grains of an average general bulk. Where 

 they are large, the cross fracture of the rock is very 



