JURA. QUAttfz ROCK. 12 19 



to class the doubtful specimen in that division which is 

 the predominant one, geologically considered. Thus the 

 occasional presence of mica will no more exclude a speci- 

 men or a bed from the series of quartz rock, than the 

 predominance of quartz will remove an occasional bed of 

 the latter from the mass of micaceous schist in which it is 

 situated. It is among specimens of this character, that the 

 substance so much used in ornaments by the name of avan- 

 turine, is found ; its colours varying* with the hues of the 

 mica and of the quartz, and the nature of its reflections 

 and lustre depending, partly upon the magnitude and 

 position of the scales of mica, and partly on the quality 

 of the quartz which unites them. The progress of this 

 substance may readily be traced from the coarsest mica- 

 ceous schist to the most brilliant specimens, and numerous 

 varieties of it may be found throughout Scotland. 



But the alternations between micaceous schist and 

 quartz rock are often more decided and on a larger scale ; 

 a sudden and complete change taking place where they 

 meet. Scarba and Jura afford abundant examples of this 

 nature, and it is also to be found in many other parts of 

 Scotland. The space occupied by the quartz rock in these 

 cases is so great, that it constitutes the chief part of the 

 series, and cannot therefore be considered as subordinate 

 to the micaceous schist. In many places indeed this 

 latter substance is altogether wanting; large tracts of 

 country, exhibiting strata of some thousand feet in thick- 

 ness, consisting solely of quartz rock ; giving it thus, as I 

 formerly remarked, a decided claim to be ranked as a 

 principal member of the primary class. 



As micaceous schist and clay slate have been proved to 

 alternate, thus quartz rock also alternates with this latter 

 substance. The mode of this alternation, like that of the 

 former, is various ; the Iamina3 being of greater or less 

 thickness. On a small scale these alternations are not 

 unfrequent ; between large beds they are rare. In some of 

 these cases the same gradation occurs between those two 



