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UNIVEESITY OF VIRGINIA PUBLICATIONS 



III. Guilford, Howard county, Maryland. Williams, G. H. 15th An. Kept., U. S. 



Geol. Survey, p. 672. 



IV. Woodstock, Howard county, ilaryland. Williams, G. H. 15th An. Kept., 



U. S. Geol. Survey, p. 672. 

 V. Sykesville, Carroll county, Maryland. Williams, G. H. 15th An. Kept., U. 

 S. Geol. Survey, p. 672. 

 VI. Rowlandsville, Cecil county, Maryland. Basoom, F. Maryland Geol. Survey, 

 Cecil County Report, 1902, p. 120. 

 VII. Dorsey Run Cut, Howard county, Maryland. C. R. Keyes, 15th An. Rept., 



U. S. Geol. Survey, 1895, p. 697. 

 VIII. Potomac Stone Company's quarry, District of Columbia. Williams, G. H. 

 15th An. Rept., U S. Geol. Survey, p. 672. 

 IX. District of Columbia. Merrill, G. P. Rocks, Rock-Weathering and Soils, New 

 York, 1906, p. 185. 



In general, the complete analyses indicate that alumina, ferric iron, and 

 the alkalies vary with the silica, while ferrous iron, lime, and magnesia vary 

 inversely. The percentage of silica (70.89 per cent, average) indicates 

 relatively acid rocks, with a wide range in the amount of quartz shown, 

 from 13.6 per cent, to 41.2 per cent., and an average of 30.25 per cent. 

 Computed from the analyses, the percentages of quartz are : I, 41.3 ; II, 34.6 ; 

 III, 28.7; IV, 28.7; IVa^ 13.6; V, 40.9; VI, 28.4; VII, 26.8; with 

 an average of 30.4. 



Computing the mineral composition of the feldspars from the percentages 

 of KjO, NaaO, and CaO given in table B, by assigning the potash to ortho- 

 clase, and the soda and lime to albite and anorthite, respectively, the analyses 

 show the constitution in table C. 



C. Table of feldspar composition of Maryland acid plutonic roclcs. 

 (Numbers correspond to table B.) 



It will be observed that the plagioclase molecules as calculated range 

 from nearly pure albite (AbijAni) to basic oligoclase (AbjAnj), with an 

 average of acid oligoclase {Ahihxi^). It is not improbable that some of the 



'Dorsey Run, Howard county. Maryland. 15th An. Rept., U. S. Geol. Survey, 

 1895, p. 722. 



^Anorthite not calculated because of unknown amounts of epidote and apatite 

 present in the rock. 



