284 Prof. J. 1). Dana on some Results of 



manifestoes of some kind of community of origin in the earth's 

 ejections, as well as announcements of the kind of material that 

 constitutes to a large degree the deeper regions of the crust. 



There is, however, considerable diversity among true igneous 

 rocks. Besides the prevalent doleritic iron-bearing kinds, inclu- 

 ding dolerite, peridotite, melaphyre, &c, to which the preceding 

 observations relate, there are trachytic or felspathic rocks, whose 

 constitution, especially in the case of the more siliceous varieties, 

 comports with their origin from the fusion of granitic rocks or 

 of sedimentary beds made of granitic material. They occur in 

 the same regions in which doleritic dikes exist, as well as among 

 volcanic products. 



Such a composition does not, however, make it certain that 

 the supercrust was their source ; for true granite and gneiss and 

 even quartzites occur constituting old Archsean terrains ; and 

 it may be that essentially the same kinds of rock are to some 

 extent represented in regions of the infra- Archsean crust and 

 even in the fire-seas. 



Basaltic and trachytic rocks often occur so combined in a 

 single volcanic mountain, that we seem forced to find some other 

 explanation of their origin than that of the fusion of unlike se- 

 dimentary strata. As long since observed by Von Buch at 



rentian) dolerite from Grenville, Canada, bv Hunt (Geol. Rep. Canada, 

 1863) :— 



G.=2*88. G.=2*96. IIL IV - 



Silica 52-13 47*80 4940 50*35 



Alumina. 14*87 14*80 14*62 1 7*35 



Sesquioxide of iron . , . . . . . . . . 12-50 1 



Protoxide of iron , 11*40 1308 16'27 



Protoxide of manganese . 0"32 0*09 



Magnesia 6*46 6'84 5*86 4*93 



Lime 10-56 12*89 10-34 10-19 



Soda 2*60 2*48 2*28 2*28 



Potash 0*69 0*86 0*34 0*69 



Volatile (H 2 O &c.) .... 1*19 1*41 2*41 0*75 



100*22 10025 101*52 99 04 



Lyell recognizes the occurrence of " trap " in all periods from the Lau- 

 rentian to the present (Principles, chap, vii.) ; and Mr. S. Allport an- 

 nounces in a recent article (Geol. Mag. May 1873), entitled "Tertiary and 

 Paleeozoic Traps," that he has nearly ready for publication the results of 

 many investigations sustaining the view which he has "for some time 

 maintained, as to the complete identity in composition and structure of 

 eruptive rocks of widely separated geological periods." 



1 Hunt says the iron is protoxide, though determined under the form 

 of sesquioxide in his analysis. 



