BY THE CONDITIONS OF THEIR EXTRUSION. 13 



in depth of rocks of average weight. Indeed such a pressure would be sufficient to squeeze 

 into movement rocks that were not nearly enough heated to flow at ordinary pressures.-* 



It is worth while to notice that this downward extension of dykes would give an 

 appearance tending to support the present opinion that dykes are derived from an 

 indefinite depth. It also tends to mask the fact of the limitation of the origin of certain 

 classes of dykes to certain horizontal areas, and it may happen, if this h3^po thesis be true, 

 that a dyke that appears to arise from the deeper regions of the earth may have been 

 derived from certain beds which have since disappeared from the region where it is found. 

 It is obvious that this will make the inquiry into the origin of dykes a matter of much 

 more difficulty than it would be if we could assume that such lavas always rose upAvard 

 from their point of origin. 



The question will naturally be asked, why, if the dyke stones are derived from sections 

 near the surface of the earth, they are not frequently seen at their point of ori^'-in? To 

 this it is a sufficient answer that we have never sought for such phenomena, and where 

 they would be found we should always have a considerable amount of disturbance that 

 would tend to make the indications of their origin difficult to decypher and not calculated 

 to arouse the attention of the observer. A thin layer of easily fusible rock might be 

 forced into the chasm formed by the opening of a fissure so as to permit the beds above 

 and below it to rest upon each other; or, if a thicker bed, it might appear as a horizontal 

 prolongation of the dyke that had been formed from it. Moreover, there would be a great 

 disturbance made, at the point where the molten rock emerged from its bed into the 

 fi.ssure, which would tend to confuse the record of the event. Despite this difficulty there 

 are some cases in which I have observed what has seemed to me fiiir evidence of the local 

 origin of certain traps. Not infrequently trap dykes may be observed to run out downwards, 

 which requires us to suppose either that they have been injected downwards, or that they 

 have originated above the point where we find them. In other cases I have observed small 

 dykes that extended downwards into a horizontally disposed mass of dyke matter and could 

 not be traced below a certain level. It is true that the difficulty which we find in deter- 

 mining the true extension of a dyke on account of its frequent doublings and twistings limits 

 the value of such evidence, yet it seems to me that, if we start without the prepossession 

 in favor of the origin of all lavas below the observable level of the earth's crust, we can 

 better reconcile the fiicts with the hypothesis of their local and sujjerficial origin than with 

 any other. 



It is worth while to notice in this connection that some of our conglomerates show 

 the effect of heat in softening the mass of the dejiosit. It often happens even when 

 the mass is apparently not much metamorphosed, that the more siliceous pebbles 

 are softened and squeezed into each other in a surprising way. I have always 

 found that the siliceous elements of the mass have been the most affected by 

 the action ; sometimes this action is limited to a slight change of form of the 

 pebbles or the indenting of one by the other, as in the Roxbury conglomerate or 



1 It seems to me tliis theory of tlie origin of dyke squeezing of matter for great distances, would in a way 



stones is better than that which we would derive from Mai- localize the heat arising from the downward falling of a great 



lett's ingenious hypothesis. While the falling together of mass of rocks upon a small bulk of materials. 

 the rocks could not develop a great amount of heat, this 



