KERRERA. GEOLOGY. 117 



consisting only in the variety, number, or magnitude of 

 the imbedded pebbles and fragments. 



The grey sandstone is fundamentally composed of 

 grains of clinkstone or compact felspar of different colours; 

 namely, of the basis of those traps which constitute the 

 present superincumbent strata of the adjacent country. 

 A mixture of hornblende, quartz, mica, and occasionally 

 of calcareous matter with clay, vary its aspect without 

 causing any material change in its fundamental character. 

 The substances imbedded in this base when it passes 

 to the state of conglomerate, are various ; more so than 

 in any other rock of similar structure that has fallen under 

 my examination in Scotland. Granite and quartz, of 

 different colours and textures, are among the most con- 

 spicuous, since they are among the most durable materials. 

 But the peculiarity here remarkable, is the great number 

 of imbedded pebbles of the various trap rocks ; by which 

 it is no less distinguishable than by the composition of 

 its base. These pebbles present every possible variety ; 

 namely, basalt, greenstone, amygdaloid s, cavernous trap, 

 and clinkstone or compact felspar ; together with those 

 porphyritic or other modifications that are found in nature 

 associated with the other rocks of this extensive family. 

 Their size is extremely various, and frequently enormous ; 

 and as they are often scattered in abundance on the shores 

 and on the surface of the land, they are sometimes readily 

 mistaken for transported alluvia, detached from their 

 original rocks without having undergone this intermediate 

 state of transmigration. 



Although the geological position of these strata is that 

 of immediate superposition to the argillaceous schist, 

 their local position is often so intricate and so difficult 

 to comprehend, that the order of their arrangement can- 

 not be conjectured without much patient research; a 

 circumstance in which this rock differs from that which 

 usually separates the primary and secondary strata ; the 

 position of this being commonly as regular as those 



