SKY. GEOLOGY. OVERLYING ROCKS. 363 



under it. Even if it were possible to define them, 

 neither the geography of the country nor the nature 

 of the map admit of accurate references. But it is 

 still possible to make some distinctions, and it will be 

 useful to make them even where they are unavoidably 

 imperfect. With this view I have attempted to sketch 

 both the characters and the places of the most important 

 or remarkable, omitting others that were of less moment 

 and that defy all arrangement. 



In conformity to the most general usage, I have 

 throughout distinguished trap from syenite, even when 

 speaking of them merely in their geological relations; 

 although there is no geological distinction between the 

 two divisions, which pass into each other and possess 

 a common relation to the stratified rocks. For the 

 same reason, porphyry, where it occurs, is also thus 

 distinguished; though more obviously a modification of 

 the same substances that constitute trap and syenite, 

 and in many places indicating its own rank as an 

 accidental variety, by the frequent renewal and alternate 

 absence of the only character by which it differs from 

 the more simple rocks that constitute its base. Geologists 

 will still consider these three divisions as appertaining 

 to the trap formation. 



Those who have been engaged in the investigation 

 of this family of rocks must be sensible that the classi- 

 fication above mentioned is imperfect, and founded on 

 improper and insufficient characters. The defect will 

 be principally felt in that division which is known most 

 particularly by the name of trap, and includes many 

 different substances associated by scarcely any bond 

 but colour. The imperfections of nomenclature which 

 result from the transition of different substances into 

 each other, will continue to exist in a certain degree 

 whatever care may hereafter be taken in separating the most 

 remarkable individuals by appropriate names. I shall 

 not here attempt to remedy this defect, sensible of my 



