2S4 Pirsson and Rice — Geology of Tripyramid Mountain. 



mostly left upon the lower slopes; and, in accordance with 

 this, we find the syenite, although appearing firm and solid in 

 the material on the Slides, yet somewhat affected by weather- 

 ing when seen under the microscope, and the monzonite very 

 much more altered, while the gabbro and norite are very fresh 

 and unchanged. 



For these reasons we do not ascribe the sheet jointing to 

 exposure, but believe that the explanation for it is to be 

 sought in the jointing phenomena incident to the cooling and 

 contraction of an intruded igneous mass. In a dome-surfaced 

 intrusion, as the planes of cooling descend into the mass, the 

 ensuing contraction would produce a sheet jointing, or 

 onion-like structure, parallel to its surface. That this is so 

 is well shown in some of the more massive of the laccoliths of 

 the west. It is not our purpose here to discuss the jointing 

 phenomena which ensue in various kinds of intrusions and 

 under various conditions, for the subject is too extensive for 

 treatment in this place ; we merely wish to point out that this 

 is one type of jointing which characterizes such intrusions, the 

 proof being not merely theoretical, but actual.* And on the 

 other hand, the possession of this kind of jointing by an igne- 

 ous mass may be legitimately used to help in deducing the 

 original form of the intrusion. 



Structure and Origin. 



From what has been said in the foregoing and by reference 

 to the geologic map, it will be seen that, accepting the field 

 observations of Hitchcock and Huntington on the eastern side, 

 we have three sections up the mountain, one on the north- 

 northwest slope, one on the south-southwest slope, and one on 

 the east ; that is roughly, in a general way, at 120° from one 

 another. Disregarding the narrow norite phase on Slide 

 Brook, each of these gives the same sequence of rock types, 

 as follows : first, the general granite of the region, which at 

 about 2,500 feet elevation is succeeded by a coarse-grained 

 black gabbro ; this gives way to monzonite, which at 3,500 feet 

 (on the North Slide) is in turn replaced by the syenite which 

 forms the upper part of Tripyramid. Thus the succession 

 reads granite, gabbro, monzonite, syenite. It is this succession 

 which led Hitchcock to draw the section we have previously 

 given and to consider the mountain as of synclinal structure. 

 It is evident from his remarks that his observations led him 

 to consider it, aside from the gabbro, as an eruptive mass. Dis- 

 regarding the views about stratification in the gabbro and syn. 



* As in Square Butte in the High wood Mts. of Montana, referred to later. 

 Conf. also Ramsay, Das Nephelinsyenitgebiet auf der Halbinsel Kola, 

 Fennia, xi, No. 2, p. 81, 1894. 



