OVERLYING AND TRAP ROCKS. 12.5 



portions of the general district formed of these strata; 

 and the more recent masses of trap which cover and 

 intersect them, are also limited to the same space. In 

 Canna and near Ohari, the rounded nodules betray 

 other marks of long exposure than their attrition, in 

 the loss of their superficial amygdaloidal nodules; 

 both facts showing that these tufos and trap alluvia, 

 originally limited to one place, have been consolidated 

 by fresh formations of trap at a distant interval of time, 

 and in the very same spots. And this conclusion is, 

 to establish an important point of resemblance between 

 the trap rocks and those of volcanic origin ; or of 

 proving, that like volcanoes, eruptions of trap have 

 been repeated at the same points. The complicated 

 successions of veins in Airdnarnurchan afford further 

 evidences of the same nature ; as similar ones might 

 be pointed out in many other places. Combined with 

 the local and limited nature of the trap rocks, these 

 facts serve to extend that identity, rather than resem- 

 blance, between volcanoes and the sources of trap, which 

 is evinced by every fact and feature in the history of 

 this family. 



If the trap rocks are generally associated with the 

 secondary strata, this rule is not exclusive, yet the con- 

 clusion to which it leads, is not very important. If 

 the secondary strata covered all the primary at the time 

 of the formation of the trap, it must necessarily be al- 

 ways thus associated ; and its present existence above 

 them only proves that it has protected them from that 

 waste which may have removed other parts of the sur- 

 face. But its contact with the primary does not prove 

 that the secondary may not have once existed even in 

 those situations; as the recent period of its formation 

 might have admitted of the intermediate removal of 

 the portions now absent in those places. 



