and of Trap in different Parts of Scotland. 237 



deep beds; and being, in other places, carried away by 

 the usual causes which transport loose materials along the 

 surface. 



Hence the granite appears at the surface in a very irregular 

 manner, and often very unexpectedly. In these cases it has 

 no relation to the form or altitude of the place where it oc- 

 curs ; since it is found in the lowest as well as the highest si- 

 tuations; in both of which also, the gneiss appears in the 

 same uncertain manner. In many places it will thus happen 

 that a patch of granite may not occupy an extent of more 

 than a few yards, or perhaps a few hundreds ; and cases of 

 this nature occur in every part of that granitic district. 



To the geologist who is anxious to lay down a district of 

 this nature in a map, this circumstance is a source of great 

 labour ; requiring all his care and industry, and demanding, 

 indeed, a degree of toil which is almost incomprehensible. 

 Here, also, the difficulty is not limited to the granite only ; 

 affecting the stratified rocks nearly in an equal degree. Where 

 strata present a considerable continuity over any tract of coun- 

 try, it is almost always easy to infer the existence of large 

 portions of them without actual examination ; by the compa- 

 rison of bearings and dips, and by general inferences respect- 

 ing their necessary connection or prolongation. But where 

 they are found forming masses so thin as barely to cover the 

 subjacent granite, not only their dips and bearings are irregu- 

 lar and uncertain, but the frequent interruption to which they 

 are exposed by the intruding granite, renders it impossible to 

 prolong them, or infer their existence in any particular spot 

 without actual examination. Thus they become as difficult of 

 investigation as the granite itself, in which the want of strati- 

 fication precludes all species of investigation, but that which 

 proceeds on the basis of actual and manual examination. 



In Aberdeenshire, other causes tend materially to increase 

 the labour of the geological surveyor. The quantity of allu- 

 vial matter arising from the decomposed gneiss is such, as of- 

 ten to cover all the rocks to a great depth ; so that it is only 

 from the most casual appearances, in the bed of a rivulet, a 

 quarry, or the side of a road, that we are enabled to discover 

 whether gneiss or granite is present, or whether sonic other of 



