74 Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



unless it be also assumed that the want of cohesion between these beds 

 is co-extensive with the injected bed, an assumption, which as I have 

 before remarked, must probably be in general considered as totally 

 inadmissible. The probable consequence of simultaneous injection from 

 different fissures, (supposhig the injected matter not in too great quan- 

 tity), would be the formation of partial and unconnected beds as repre- 

 sented in the annexed diagram. 



For these reasons then we cannot hesitate, I think, to conclude, 

 when we consider the general structure of stratified masses, that the 

 absence of numerous trap-veins and dykes, having their origin in the 

 upper surface of a horixontal heel of trap, with the want also of very 

 frequent indications of violent mechanical action in the lower portion 

 of the superincHmhent mass, affords indubitable proof of the fact of such 

 liorizontal bed having been ejected over the exterior surface existing 

 at the time of its eruption. 



79. The existence of a single vein or dyke sucli as above described, 

 in rocks incumbent on a horizontal bed of trap, is clearly an indubitable 

 proof of injection ; but it must not therefore be concluded, that every 

 trap-vein or dyke in the superincumbent strata affords this unequivocal 

 testimony, since it is manifest that such a vein or dyke might possibly 

 be produced by injection, subsequently to the formation of the hori- 

 zontal bed, which it may have traversed exactly in the same manner 

 as any other stratum*. The decisive character of the evidence of injec- 

 tion afforded by a vein, consists in its originating in the upper surface 

 of the injected bed. We may also remark, that indications of mechanical 

 action on the beds beneath a bed of trap will not necessarily afford con- 

 clusive testimony as to the fact of injection, because such appearances 

 miglit be produced, to a certain extent, by the force of an ejected, as 

 well as of an injected bed. It is in the superincumbent beds that we 

 must seek for the evidence in question. 



80. It is not my object to enter into any detailed comparison between 

 observed facts, and these theoretical deductions, but I think it necessary 



* Many instances are given by M'Culloch of veins of trap existing in trap. See " Descrip- 

 tion of the Western Islands." 



