Granites of Cecil County, in North-eastern Maryland. in 



hence approach most closely to the type of the normal and 

 undoubtedly igneous granites. In reviewing the evidence 

 now available as to the original character of these rocks, we 

 may conveniently arrange the matter under several heads. 

 We shall, therefore, summarize in succession the proofs of 

 igneous origin based upon, i, chemical composition; 2, 

 mineralogical composition and structure ; 3, geological mode 

 of occurrence; 4, contact action. 



1 . Chemical Composition. — While the two analyses made of 

 the Rowlandville and Port Deposit granites differ considerably 

 in their silica percentage, they both agree in the proportion 

 of their chemical elements with igneous granites ; hence, 

 while this of itself can not be regarded as a definite proof, it 

 is in accord with the theory of igneous origin, since, as 

 Rosenbusch* has pointed out, it is only by an accident that 

 sedimentary deposits may have the bulk composition of 

 igneous masses. As a rule, such sedimentary deposits would 

 vary considerably from eruptive types, and as there is reason 

 to believe that the bulk composition of a rock is not greatly 

 changed by its metamorphism, we may regard the chemical 

 analyses of the Cecil County granites as at least indicative of 

 their igneous origin. 



2. Mineralogical Composition and Structure. — The minerals 

 which compose the rocks under consideration are those 

 which most commonly make up the substance of normal 

 eruptive granites. Feldspar, quartz and biotite are their 

 essential constituents in about the same proportion as are 

 ordinarily found. They are free from those minerals like 

 andalusite, cyanite, staurolite, cordierite, etc., which are so 

 characteristic of metamorphosed sediments, while garnet is 

 known to be readily produced by the metamorphism of 

 igneous as well as of sedimentary material. 



Allanitc is a widely distributed constituent in these rocks, 

 both in the form of crystals and small grains. While the 

 presence of this material was formerly regarded as good 

 proof against eruptive origin, because it was thought to be 

 unable to stand high heat, the investigations of Scheerer, 

 Daubree and others, have shown the fallacy of this argument. 



*Zur Auffassung der chemischen Xatur des Grundgebirges Tscher Min. und 

 Petrog Mitth., Vol. 12, p. 49, 1891. 



