Granite of Aberdeenshire, 41 



been esteemed a true stratification, are however not uncommon 

 among the rocks of this family ; but these, whatever resemblance 

 they may bear to genuine stratification, admit of other explana- 

 tions. Into the details of these it is also impossible here to 

 enter, as it would involve a long train of facts and discussions. 

 It must suffice to say, that all the instances of stratified trap 

 yet produced may be easily explained, and arc, indeed, in most 

 cases, demonstrably proved to be either veins parallel to the 

 strata in which they lie, or thin superincumbent masses of 

 which the forms have been determined by those of the sub- 

 jacent stratified rocks ; or else strata of shale, or of other sub- 

 stances, which have been converted into trap by the same 

 causes which sometimes change them into siliceous schist ; or, 

 lastly, tufaceous rocks which appear to have been either de- 

 posited in the shape of mud, as similar materials so frequently 

 are by volcanic eruptions, or else generated, like the sandstones, 

 from the wear of more ancient rocks of the same nature. 



Granite, and the trap rocks, are both found in the shape of 

 veins, and they are the only rocks which are known to be dis- 

 posed in this manner, it being here understood that, under the 

 term trapj is included every instance of porphyry, as well as 

 those varieties which are peculiarly connected with the most 

 recent greenstones, basalts, or claystones. It has indeed been 

 asserted, that sandstone and limestone, and even clay-slate, 

 have been found forming veins, but it is easy to see that these 

 imaginary observations are either the result of ignorance or 

 inexperience, or are the produce of something more than volun- 

 tary self-deception for the purpose of supporting an hypothesis. 



The veins, both of granite and of trap, have, in so many 

 instances been traced to principal masses of the same rocks 

 as to leave no reason to doubt that this character is, in both, 

 universal. In both cases the want of free access occasionally 

 prevents these connexions from being ascertained ; in the trap- 

 rocks another cause sometimes interferes with this investigation, 

 namely, the entire loss, from the effects of time, of the great 

 superincumbent masses, while the veins remain, protected from 

 destruction by the strata in which they lie. 



