in Pennsylvania and Maryland. 495 



This rare mineral is so abundant and occurs in such interesting 

 relations that it will soon be made the subject of a special com- 

 munication. 



The manner in which these compact, fine-grained rocks have 

 been reduced along lines of shearing into fissile slates is most 

 instructive. This is accompanied by the abundant production 

 of sericite in the acid, and of chlorite in the basic rocks, while 

 the phenocrysts and all original structures are obliterated. 

 Amygdules, where present, usually remain as flattened spots, 

 either lighter or darker than the rock. Several suites of speci- 

 mens have been collected to illustrate this process, but the 

 account of its details must be reserved for a future paper. 



That the large area of fissile, pale green schists occurring 

 between Pine Grove Furnace and Laurel Forge consists of 

 sheared felsite may be seen from the following analysis, made 

 for commercial purposes by A. S. McCreath. This rock, which 

 is locally known as "soapstone," is extensively used at Pine 

 Grove Furnace in the manufacture of brick. Its alkalies were 

 not determined. 



Si0 2 



A1.0, 



FeO . 



CaO 



MgO 



Alkalies . : 



lorn. 



'74-9V0 



13-860 



2-700 



0-220 



1-230 



9 



2-058 



Total 95-038 



6. Comparison with other regions. — The comparative rarity 

 of very ancient volcanic rocks in America as compared with 

 Great Britain* and other parts of Europe is doubtless due to 

 their not having been recognized, rather than to their actual 

 absence. The opinions entertained by Hunt of rocks like those 

 of South Mountain have greatly retarded, in this country, the 

 appreciation of their true character. Still they are well known 

 on Lake Superior and in Missouri through the writings of 

 Irvingf and E. Haworth \\ Wadsworth§ and Dillerfl have 

 described them in eastern Massachusetts, Shaler^f in Maine, 

 and the northern continuation of this same belt has been made 



* Sir A. Geikie : Anniversary Address, Quart. Jour. Geol. Soc , vols, xlvii and 

 xlviii, 1891-1892. 



f Monographs U. 8. Geol. Survey, v, 1883: and Bulletin, No. 62, 1890. 



% Am. Geologist, vol. i, p. 280, 1888, and Bull. Missouri Geol. Survey, No. 5. 



§ Bull. Mus. Compar. Zool Cambridge, vol. v, No. 13, p. 282. 



|| Ibid., vol. vii, No. 2, 1881. 



«|[ This Journal (III), vol. xxxii, p. 40, 1886. Ann. Rep. IT. S Geol. Surv., vol. 

 viii, p. 1043, 1889. 



Am. Jour. Sci. — Third Series, Vol. XLIV, No. 264. — December, 1892. 

 33 



