GEOLOGIC POSITION AND STRUCTURE OF MASS 391 



views very briefly, and it is in these respects only that I find reason to 

 differ with him. 



A rough geological map, based on that of Williams, is given, as refer- 

 ence to it is necessary for proper understanding of the main questions. 

 It only gives the main features and larger areas, with some of the dikes, 

 while the contour lines have been omitted, so that for details occasion- 

 ally mentioned the reader must consult the original. 



GEOLOGIC POSITION 



The igneous rocks of Magnet Cove are situated in an area of much 

 folded Carboniferous shales and sandstones south of the Lower Silurian 

 area, which extends from Hot Springs to Little Rock, and on the south- 

 eastern border of the Ouachita uplift. It is surrounded at some distance 

 on the north, east and south, and partly on the west, by novaculite 

 ridges forming part of the peculiar zigzags described by Griswold.* 



The comparatively late age of these rocks was first determined by 

 Branner f and confirmed by Griswold % and Williams. § The last says : 

 "All the igneous rocks are younger than the surrounding Paleozoic rocks 

 and have forced their way into them. They were formed after the fold- 

 ing and bending and after some of the erosion of the Paleozoic rock had 

 been accomplished, probably during late Cretaceous times." Branner 

 has lately || expressed the opinion that the Arkansas syenite intrusions 

 are of Tertiary age. It is only necessary to say that my observations 

 fully confirm the view that the intrusions were subsequent to the dis- 

 turbance of the surrounding Paleozoic rocks, and also Williams' state- 

 ment^ that all the igneous rocks are undoubtedly intrusive in charac- 

 ter, and that no extrusive flows or ash-beds were observed. 



STRUCTURE OF THE MASS. 



Obstacles to observation. — Unfortunately, the conditions are far from 

 ideal for the study of the structure and of the relationships of the 

 igneous rocks to each other and to the surrounding sedimentaries. The 

 greater part of the area, especially the annular " Ridge " and the igneous 

 patches elsewhere, is densely wooded and covered deeply with leaves 

 and soil, while in the " Cove " basin, most of which is under cultivation, 

 the accumulation of detritus and the products of decomposition of the 



*L. S. Griswold: Novaeulites of Arkansas. Ann. Rept. Geol. Survey of Arkansas for 1890, vol. 

 iii, chap. xv. 

 f J. C. Branner, in Proc. Amer. Ass. Adv. Sci., vol. xxxvii, 1888, p., 188, 

 % Griswold : Op. eit., p. 321. 

 I Williams : Op. eit., p. 342. 

 || Am. Jour. Sci., vol. iv, 1897, p. 365. 

 If Williams : Op. eit., p. 343. 



