390 H. S. WASHINGTON — IGNEOUS COMPLEX OF MAGNET COVE 



Part I. — Geologic Structure of the Complex 



INTRODUCTORY 



The igneous rocks of Arkansas, especially those of Magnet Cove, have 

 heconie classical through the careful and detailed work of the late J. F. 

 Williams, whose volume* is, of course, well known to all petrographers. 

 Study of his work, and especially of the map which he gives, led me to 

 think that the structure of the mass and the relationships of the various 

 rocks were not such as were hriefly indicated by him, as will be shown 

 later, but that the complex forms an excellent, though peculiar, example 

 of a highly differentiated mass of magma, probably a laccolith, the 

 rocks of which form a series of very interesting types. 



It must be remembered that at the time Williams wrote his report 

 the notion of laccoliths had not become widely accepted, especially in 

 Germany, where he had studied. Furthermore, at this time none of the 

 most striking and now well known examples of differentiated laccolithic 

 masses had been described. Indeed, the idea of the differentiation of 

 magmas was only beginning to take definite shape, the classical paper 

 of Rosenbusch f dating from 1889, the work of Brogger J on the Christi- 

 ania region appearing in 1890, and that of Iddings§ in 1892. It is 

 therefore not surprising, and does not detract from the high standard of 

 Williams' work, that such topics are omitted from his discussion, and 

 that he barely refers || to the differentiation of the magmas. 



In the spring of 1899 I had the opportunity of spending a few days 

 at Magnet Cove. While the results did not come up to my expectations 

 in all respects, yet my observations confirmed me in my idea, and they, 

 with the material collected, enabled me to examine the subject on my 

 own account, the results of which study I now purpose to give. 



The descriptions of the rocks by Williams are so detailed and com- 

 plete that scarcely anything can be added to our knowledge of them in 

 this direction. The analyses he gives are numerous, covering most of 

 the types, and are satisfactory in most cases; so that here also but few 

 additions are needed. It is therefore chiefly with the structure of the 

 mass and the relations of the various rocks that we shall concern our- 

 selves, since on these two important points Williams expressed his 



*J. F. Williams : "The Igneous Rocks of Arkansas." Ann. Rept. Geol. Survey of Arkansas for 

 1890. Little Rock, 1891. 

 t Rosenbusch, in Tseh. Min. Pet, Mitth., vol. xi, 1889, p. 144. 

 t W. C. Brogger, in Zeit. Kryst., vol. xvi, 1890. 

 gj. 1*. Iddings, in Bull. Phil. Spe. Wash., vol. xii, 1892, ]>. 90. 

 Williams, op. cit., )>. .'!. 



