Mr HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 71 



the consequent contraction when the mass becomes fractured by too 

 great an extension. 



^. On the Formation of Horixontcd Beds of Trap — By Ejection — By 

 Injection — Remarks on some Phenomena observed by 3I'CuUoch — 

 Effect of imperfect Fluidity in Horizontal Injections. 



76. If the quantity of fluid matter forced into these fissures be more 

 than they can contain, it will of course be ejected over the surface ; 

 and if this ejection take place from a considerable number of fissures, 

 and over a tolerably even surface, it is easy to conceive the formation 

 of a bed of the ejected matter of moderate and tolerably uniform thick- 

 ness, and of any extent. If the ejection take place over a level surface, 

 these properties of the resulting bed would seem to require a number 

 of points or lines of ejection as a necessary condition, on account of 

 the imperfect fluidity, which, according to analogy, we ought probably 

 to assign to the ejected matter. If there were only a single center of 

 eruption, a bed of such matter approximating to uniformity of thick- 

 ness, could only be produced on a surface of a conical form, having 

 the point of eruption at its vertex, and an angular elevation depending 

 on the degree in which the fluidity of the ejected mass should difler 

 from perfect fluidity. Where no such tendency to this conical structure 

 can be traced, it would probably be in vain to look for any single 

 center of ejection. On the supposition too, of ejection through con- 

 tinued fissures, or from a number of points, that minor unevenness of 

 surface which must probably have existed under all circumstances during 

 the formation of the earth's crust, would not necessarily destroy the 

 continuity of a comparatively thin extensive bed of the ejected matter, 

 in the same degree in which it would inevitably produce that effect 

 in the case of central ejection. 



77. I will now proceed to consider the formation of a horizontal 

 bed by injection ; what limits may be imposed on the probable or possible 

 extent of it, and with what phenomena it may be accompanied, which 

 may serve as tests for distinguishing a bed so formed from one formed 

 by ejection over the external surface. 



