DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 121 



I have already mentioned my belief in the fluidity of all 

 except the acid magmas under moderate subterranean 

 pressure; and, if this be granted, the sinking of the earlier 

 formed crystals, rendering more basic the lower part of a 

 mass, may be regarded as being probably a not uncommon 

 type of differentiation. It has been described by Darwin 1 

 and Iddings. I venture to suggest that the familiar basic 

 concretions in granites have been formed in this way. If a 

 layer of small crystals had hardened over the bottom of a 

 magma cavity, or in the duct which fed it, a further intru- 

 sion of magma, such as probably frequently occurs, would 

 breakup this hard layer and partially incorporate the frag- 

 ments, leaving subangular pieces such as we find. 



It should be remembered that under certain circum- 

 stances the sinking of heavier constituents either during 

 crystallisation, or by liquation, can produce both the 

 appearance of an acid centre bordered by basic rock, and 

 the reverse, namely a basic centre and relatively acid 

 periphery, the actual appearance depending on the extent 

 of denudation. Some of the intruded magma cools on the 

 walls of the cavity before differentiation takes place, then 

 the heavier constituents sink to the bottom, and the lighter 

 magma remaining (usually more acid) occupies the centre 

 of the upper part of the cavity. According to whether 

 denudation has drawn the surface along the upper portion 

 or the lower portion, so will the centre be more acid or more 

 basic than the border. It is possible that the laccolite of 

 Ramn'as 2 is an example of the latter, whilst the nepheline- 

 syenite-shonkinite laccolite of Square Butte, Montana, 3 is 

 probably an example of an acid centre produced in this 

 way, the mineral differences indicating liquation as the 

 probable process of differentiation in the latter case. 



1 Volcanic Islands (1844). 



2 Brogger, Zeit. f. Kryst., xvi, (1890) p. 45 ; akeritein centre (Si0 2 58'5) 

 quartz porphyry at periphery (Si0 2 71"5). 



3 Bull. Geol. Soc. Amer., vi, (1895) p. 389. 



