Revieics — Dr. Wa>ihui(jton — Igneuus Rocks, Arkansas. 177 



III. — Igneous Cobiplex of Magnet Cove, Arkansas. By Heney S. 



Washington. Ball. Geol. Soc. Amer., vol. xi, pp. 389-416 ; 1900. 



The Foyaite-Ijolite Skuies of Magnet Cove: A Chemical 



Study in Differentiation. By Henry S. Washington. 



Journ. Geol., vol. ix, pp. 607-622, 645-670 ; 1901. 



THE great variety of igneous rocks, particularly of types rich in 

 alkalies, developed at numerous centres in the eastern half of 

 the North American continent, promise to yield many results of 

 more than local interest to the petrologist, and Dr. Washington has 

 now been engaged for some years in the stud}'^ of certain important 

 districts. From Essex County, Massachusetts, he has passed to 

 the no less interesting area of Magnet Cove, Arkansas, originally 

 described by the late J. F. Williams ; and the results of his work 

 are embodied in the two papers cited above. They include, firstly, 

 an attempt to decipher the form and geological relations of the 

 intrusive bodies, a matter of some difficulty owing to the poorness 

 of the exposures ; secondly, a re-examination of the rocks themselves, 

 including a number of new chemical analyses, with calculations 

 of the percentage mineral compositions and comparison with known 

 types from other districts; and, finally, a chemical study of the 

 principal rock-types as a natural group, with reference to their 

 common origin by magmatic differentiation. 



The outcrops of the chief igneous rocks of the Cove occupy an 

 elliptic area, and divide, as shown on Williams' map, into an inner 

 ellipse, composed of the most basic rocks, and an outer elliptic 

 ring, consisting of rocks on the whole less basic and separated 

 from the former by a ring of metamorphosed sediments. Williams 

 believed the outer ring of igneous rocks to be of distinct origin from 

 the inner body and of decidedly posterior intrusion, but Washington 

 considers the whole to form a single group with intimate genetic 

 relationship. The latter author, indeed, makes the outer ring 

 continuous underground with the inner mass, the intervening meta- 

 morphosed sediments resting on the igneous rocks and representing 

 the relics of what was once the cover of a single laccolitic intrusion. 

 The argument here is scarcely convincing : although the meta- 

 morphosed rocks form a broken ridge, the area is not one of high 

 relief, and it is noteworthy that none of the igneous rocks belonging 

 to the outer ring are found, even as outliers, in the inner part of the 

 Cove. Disregarding the metamorphosed rocks of the ridge, the 

 various types met with are arranged roughly in concentric zones of 

 decreasing basicity from centre to margin. Assuming the centre of 

 the exposed surface to represent the centre of the laccolite, 

 Washington points out that this disposition of the various rocks is 

 the reverse of that described in some other igneous complexes, 

 where the most basic type occupies the outermost ring. This 

 evidently depends upon the interpretation adopted of the general 

 structure of the mass, and an alternative reading of the observations 

 does not seem to be excluded. The arrangement is apparently 

 that of a dome. If we suppose the igneous rocks to have been 



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