314 PROCEEDINGS OF THE AME:RICAN ACADEMY. 



assume that those types which Avere tniely ^n-adational in character 

 have now disappeared from sight. It is the writer's opinion, however, 

 that there always existed a sharp or very sudden change from the 

 rhom])enporph\ry into tlie other types — a practical discontinuity. 

 A number of similar sudden or sharp contacts between the peripheral 

 differentiates of a magma and tlae remaining portions have been 

 described from other localities, among the better known examples 

 being those described by Weed & Pirsson in the Shonkin Sag Laccolith, 

 Montana,'^^ by Weed & Pirsson in the Square Butte Laccolith, 

 Montana ^^ and by Pirsson and Rice at Tripyamid Mountain, N. H.'^^ 

 In such cases the evidence all points to a strong diffusion of mineral 

 compounds ''*, having an earlier period of formation, toward the cool- 

 ing surface followed by crystallization of the magma thus formed. 

 Movement of the remaining magma against these differentiated 

 phases does not seem sufficient to fully account for the sudden or 

 even sharp change from one to the other, though as in the present 

 case, it may account for a certain portion of the contacts. Assuming 

 such a diffusion toward the cooling surface to have taken place, may 

 not the process ha\-e tended to overrun itself, so to speak, thus 

 bringing about a condition of supersaturation which resulted finally 

 in a more or less sharp pause in the process, a pause analogous to that 

 which has been described as marking the end of the phenocrystalline. 

 stage of growth in the case of the feldspar phenocrysts of the por- 

 phyries of this area. The result of such a pause is a practically dis- 

 continuous contact. It is not the same thing as the formation of 

 immiscible liquid phases in a silicate magma, but the final result is 

 not very different. 



The Origin of the Cognate XeneAiihs. — The opinion has been ex- 

 pressed that the patches of different texture and more basic composi- 

 tion which occur in the granite and to some extent in the porphyry 

 are in the nature of inclusions of differentiated, peripherial portions 

 of thebatholith, and they have been called cognate xenoliths. That 

 some of these xenoliths, as for example those found about tlie rhom- 

 benporphyry on Pine Hill, are nothing but fragments of the rhomben- 

 porphyry Is perfectly certain, and it may be safely inferred that many 

 others are of the same origin, though they are not so obviously con- 



7lAmer. Jour. Sd., 12, p. 351 (1S9G). 



72 Bull. Geol. Soc. Amer., 6, pp. 404-5 (1895). 



73 Amer. Jour. Sci., 31 (April, May, 1911). 



74 In this connection see also Brogger's conclusions to the same effect in a 

 paper on the basic rocks of Gran. Quart. Jour. Geol. Soc, 1, p. 15 (1894). 



