OVERLYING AND TRAP ROCKS. 131 



dently disappeared over the remainder of this tract. 

 But such superficial connexions, visible or inferred, 

 are by no means necessary, since, as in granite, many 

 trap veins are connected with deep-seated bodies of 

 rock only ; as they sometimes also are with super- 

 ficial masses at the same time ; the vein uniting the 

 productive source, in this case, with the portion 

 which has overflowed. This could riot be otherwise, 

 from the very theory ; and the real, prime source of 

 the veins is essentially below, though the apparent 

 and immediate one should be in a superficial mass. 

 Hence there may be trap veins which never were con- 

 nected with a superincumbent mass ; since it is not 

 necessary that even a volcanic lava, splitting the sur- 

 rounding strata, should become an open stream. It 

 is almost unnecessary to say that these veins cross the 

 strata at every possible angle ; but these being gene- 

 rally high, and very commonly vertical, it is an addi- 

 tional indication of their probable origin in deep sub- 

 jacent masses. 



The matter of the veins is nearly as various as that 

 of the rocks already enumerated ; every variety 

 having occurred to me under this form, except hy- 

 persthene rock. Yet the observations on this sub- 

 stance, as yet limited to my own, are not sufficiently 

 extensive to allow of so improbable a negative; and 

 it will doubtless hereafter be proved obedient to the 

 general laws. Had Sky been other than it is, I 

 would have found these veins long ago. If, in general, 

 there is but one mineral variety contained in a given 

 vein, there are exceptions to this rule ; as I have for- 

 merly pointed out veins containing basalt, greenstone, 

 porphyry, and amygdaloid, placed in parallel laminae, 

 with others where these various substances were inter- 

 mixed in an irregular manner. 



K 2 



