5240 Dr MacCulloch on the Distribution of Granite 



surface, when compared with the surrounding country, by 

 which it may be known even at a distance ; and thus the geo- 

 logist may escape a considerable degree of the labour of mi- 

 nute investigation. But these features are by no means uni- 

 versal; nor is it often possible to form the slightest conjecture 

 respecting the presence of these rocks except by careful man- 

 ual investigation. For example, it does not always occupy 

 the hills, or even the insulated summits, when it occurs in 

 sandstone countries. On the contrary, it is by no means un- 

 usual for the trap to be found in the lowest parts of such a 

 country, while the sandstone occupies the highest ; and, of 

 this, Fife presents abundant examples. Neither is it unusual 

 for the stratified rocks to assume the external outline of trap, 

 while this rock is totally void of characteristic features. 



It is also usual in Scotland for trap to occur in connection 

 with the secondary strata, and the particular tracts where it is 

 most abundant are well known. There the investigation, at 

 least of larger masses, is comparatively easy ; because the sub- 

 stance is expected, and because the contrast which it presents 

 to the sandstones to which it approximates is commonly very 

 strongly marked. But it is found also in many places, and 

 often in an insulated manner, in the primary districts, where, 

 from previous general experience, it would scarcely be ex- 

 pected to exist. There also its general external characters are 

 less strongly contrasted with those of the surrounding rocks, 

 and thus it may escape the notice of a superficial or hasty ob- 

 server. 



These are the causes which not only render the investiga- 

 tion of a country of trap in itself difficult, but which produce 

 equal difficulty in examining the stratified rocks of any district 

 in which even one insulated mass of this rock has been found. 

 The observer there loses all confidence in his power of infer- 

 ring the existence of the stratified rocks in any place where he 

 has not actually observed them, because he is never certain 

 that some insulated mass of trap may not occur among them. 

 Thus, as in the case of granite, every spot must be traversed 

 and examined in a critical manner, if he would attain that ac- 

 curacy without which a geological survey is scarcely of any 

 value. 



