INTRODUCTION 



The writer became interested m the plutonic acid igneous rocks dis- 

 cussed in this paper, more than ten years ago, while studying those of 

 Georgia for the State Geological Survey. Since the completion of the 

 Georgia work, he has extended his studies of these rocks at intervals into 

 each of the states south of Pennsylvania. The results of these studies have 

 been published as reports by a number of the State Geological Surveys^ 

 and in various scientific journals. Within the Appalachian region south 

 of the Susquehanna river, these rocks constitute commercially an important 

 quarrying industry, amounting at present to about ?0 per cent, of the 

 total production of the United States. 



The principal results thus far obtained from the investigation of these 

 rocks briefly stated are: 



(1) Widespread development of intruded acid granular rocks through- 

 out the region of old metamorphic cr^^stalline rocks, which extend northward 

 from Alabama to New England, as extensive, irregular bodies, and a con- 

 siderable development of more basic intruded rocks, chiefly of the gabbro 

 family, including typically gabbros, diabases, pyroxenites, dunite, and other 

 peridotites. In general chemical character the acid rocks show high silica 

 and alkalies, and usually low lime, magnesia, and iron, while the basic rocks 

 show low silica and alkalies, and Hgh lime, magnesia, and iron. Of 

 the alkalies in the acid rocks, soda is quite generally high, being molec- 

 ularly equal to or greater than potash in nearly every instance. These rocks 

 have been derived from a common parent body of magma intruded, in most 

 eases, at different times. Some areas strongly suggest differentiation of the 

 magma in place", with usually a basic margin and an acid centre. The 

 sequence in igneous . activity for the Maryland portion of the region indi- 

 cates that the intrusions of gabbro were earlier than those of the numerous 

 large bodies of granite". To the south of Maryland, the relations are less 

 plain and are more difficult of interpretation. 



^A bulletin on the granites of the Appalachian region south of the Susquehanna 

 river, prepared by the writer for the United States Geological Survey, is in press. 



"Diflferentiation prior to intrusion Brogger terms primary or "deep-magmatie." 

 and differentiation subsequent to intrusion, i. e. in place, he terms secondary or 

 "laccolitic" differentiation. Quoted by A. Barker, The Natural History of Igneous 

 Rocks, 1909, p. 133. 



'Mathews, E. B. Amer. Journ. Science, 1904, Vol. XVII, pp. 141-159. 



Granite is used in this paper in its restricted sense to designate a granitoid, 

 igneous rock composed of quartz, potash feldspar (orthoclase "or microeline. or 

 both), more or less oligoclase, and the micas (biotite or muscovite, or both). The 

 first three may be combined with either of the micas alone, with hornblende, or with 

 augite. Kemp, J. F., Handbook of Rocks. Third Edition revised, 1906. p. 1S9. 



