Pirsson and Rice — Geology of Tripyramid Mountain. 287 



have been subsequently domed by quaquaversal uplift and then 

 eroded, thus producing the concentric arrangement. 



In this connection the nature of the contacts, or of the tran- 

 sition of one type of rock in the complex into another type, is 

 of importance though generally little is said upon this matter. 

 At first thought it might appear that, if the intrusion were a 

 laccolith in which differentiation had taken place in situ after 

 the magma had come to rest, the transition of one type into 

 another would be gradual and extended over some distance, 

 while if the change of one kind of rock into another takes 

 place abruptly, as in an ordinary contact, this would be 

 evidence that the intrusions of magma were successive. The 

 matter is not, however, so simple as might be inferred from 

 the above statement. Cases of transition there undoubtedly 

 are, but at Square Butte the transition zone reduces to a few 

 inches, or even less, while in the Shonkin Sag laccolith, one of 

 the most evident instances of differentiation in place that we 

 know of, the change is very abrupt and quite like an ordinary con- 

 tact, except, of course, that there are no endomorphic effects in 

 either type. It is conceivable that differentiation in the liquid 

 form may be so complete as to yield fluid masses which come 

 together in contact with definite surfaces, like oil and water 

 for example. And on the other hand, if the differentiation is due 

 to fractional crystallization about the borders of a closed cham- 

 ber, it might happen that a certain set of components would 

 separate out until a definite relation, possibly eutectic, was 

 established among those composing the rest of the fluid, where- 

 upon an abrupt change in the crystallizing minerals would 

 occur. 



Therefore, to our minds, the fact that the transitions from 

 one rock type to the other in the Tripyramid complex are 

 abrupt as in ordinary contacts is not in itself an absolute proof 

 that the differentiation did not take place in situ, or that there 

 must have been successive intrusions of magmas of different 

 composition from below upward, although they naturally sug- 

 gest this. 



Considering first some of the hypotheses that have been 

 offered to explain concentric igneous masses, it appears that 

 Harker's suggestion that this arrangement at Magnet Cove 

 may be .due to the doming and erosion of superposed sheets is 

 not applicable to Tripyramid for reasons already mentioned, 

 namely, the attitude of the contact planes, and the common 

 jointing, and also the following one. Accepting this view the 

 gabbro would be the top sheet, next the monzonite and the 

 syenite at the bottom. Now the gabbro is coarse-grained, the 

 monzonite medium or fine-grained, the syenite, by comparison 

 with the gabbro, fine-grained ; and we should thus be presented 



