ZONAL ARRANGEMENT 407 



It is to be inferred from this, I think, that the tinguaites and neph- 

 eline-porphyry are aschistic* dikes — that is, undifferentiated injections 

 of the still fluid differential zones into the surrounding rock — while the 

 fourchites, etcetera, are diaschistic dikes — that is, products of a still fur- 

 ther differentiation of these zones. 



THE ZONAL ARRANGEMENT 



Peculiarity of order of rocks. — Turning to the question of the order of 

 arrangement, it will be seen that a very peculiar feature of this complex 

 is that this differs from nearly all other laccolithic and other masses 

 of igneous rock in the order in which the component rocks occur. In 

 some laccoliths, such as those of Colorado, Utah, and Arizona, described 

 by Gilbertf and Cross, J and those of the Judith, § Castle, || and Little 

 Belt^f mountains in Montana, described by Weed and Pirsson, the 

 igneous mass is of practically uniform composition throughout. The 

 rocks of all these, it is to be noted, are granite and syenite porphyries 

 and porphyrites, all decidedly acid rocks and not very highly alkaline 

 in character. 



In others, as Predazzo,** Brandberget,tt Garabal hill,Ji Carrock fell,§§ 

 and Square butte, |[ | Yogo peak, ^[^[ and Bearpaw peak, *** in Montana, 

 composed either of rather basic or rather alkaline rocks, where the 

 magma has differentiated after intrusion, the border zone is basic and 

 the interior more acid. The same relations are observed in many com- 

 posite dikes. These cases have led us to believe that the oxides of iron, 

 magnesium, and calcium tend to diffuse toward the cooling surface, 

 while the alumina, alkalies, and silica tend to remain in the hotter 

 (central) part. 



At Magnet Cove we find exactly the contrary. The iron oxides, mag- 

 nesia, and lime are most abundant at the center, and silica, alumina, and 

 alkalies at the borders. The same oxides are differentiated, but their 

 direction of motion, if we may so express it, is reversed. 



*Brogger: Eruptgesteine des Kristianiagebietes, vol. i, 1894, p. 125; cf. Washington: Jour, of 

 Geol., vol. vii, 1899, p. 472. 

 f Gilbert: Geology of Henry Mountains, Washington, 1877. 

 { Cross : Fourteenth Ann. Rept. U. S. Geological Survey, 1895, p. 157.» 

 § Weed and Pirsson : Eighteenth Ann. Rept. U. S. Geological Survey, 1898, pt. iii, p. 437. 

 || Weed and Pirsson : Bull. no. 139, U. S. Geological Survey, 1896. 

 If Weed and Pirsson : Twentieth Ann. Rept. U. S. Geol. Survey, 1900, pt. iii, p. 562. 

 **Brogger : Eruptivgesteine Krist. gebictes, vol. ii, 1895, p. 66. 

 ff Brogger, in Quar. Jour. Geol. Soc, vol. 1, 1894, p. 31. 

 XX Dakyns and Teall, in Quar. Jour. Geol. Soc, vol. xlviii, 1892, p. 104. 

 §§ Harker, in Quar. Jour. Geol. Soc, vol. 1, 1894, p. 311, and vol. Ii, 1895, p. 125. 

 || || Weed and Pirsson, in Bull. Geol. Soc. Am., vol. 6, 1895, p. 389. 

 ffif Weed and Pirsson, in Am. Jour. Sci., vol. 1, 1895, p. 467. 

 *** Weed and Pirsson, in Am. Jour, Sci., vol, i, 1896, p. 351, 



