24 IOWA ACADEMY OF SCIENCES. 



the appearances of the granite itself, but under the touch it falls readily into sand. 

 This loose arenaceous material is without boulders or pebbles; is easily excavated 

 to a depth of many feet, and when suitably exposed to the action of running water 

 is carried away as the sand along some water-course melts into the flowing stream. 

 The absence of glacial action in Maryland has thus left many phases of rock 

 decay not usually met with in crystalline districts to the northward. In those 

 areas where ice has acted vigorously in great masses the rock surfaces are often 

 smooth and hard, with little or no traces of internal disintegration. South of the 

 terminal margin of the last great glacial invasion, as in the state just mentioned, 

 it is not uncommon to find in granitic and gneissic areas the rock broken down for 

 many feet below the surface— the debris still remainiiag in situ. The depth to 

 which this kind of rock decay takes place appears to increase rapidly in proportion 

 to the distance passed over southward from the glaciated region, as has just been 

 shown by Russell.* 



SOME AMERICAN ERUPTIVE GRANITES. 



BY CHARLES ROLLIN KEYES. 



Two hypotheses, diametrically opposed, have been entertained by geologists in 

 explaining the origin of large granitic masses. According|to the one a granite is 

 the last stage in the metamorphic alteration of mechanical sediments. Accordinsr 

 to the other a granitic mass is the product of the gradual cooling of an acidic 

 molten magma under pressure. In the first case it is claimed that all the grada- 

 tions have been traced in the same mas-i from undoubted elastics through slaty, 

 schistose and gneissic phases to the truly granitic types; so that one end of an 

 originally sedimentary deposit may be now unaltered, while the other end is a true 

 holo-crystalline rock. On the other hand unquestionable eruptive granites are 

 known to pass into gneiss and even mto the schistose stages, through the agency 

 of enormous compression. Regarding the facts deduced in the support of the first 

 assumption and without referring to any specific instances, it seems quite probable 

 that in the majority of cases bearing directly on this point sufficient discrimination 

 has not been exercised along the lines separating the semi from the holo-crystalline 

 areas. 



Recently great stress has been laid on the metamorphosing influences of oro- 

 graphic movements in disguising the original character of rocks— making erupt- 

 ives more and more like sedimentary deposits, and clastic beds more like massives. 

 But without entering into a discussion of the general subject it is intended here 

 to merely set forth some of the proofs that point to the eruptive origin of certain 

 granites of Maryland. That these particular rocks are really eruptive in character 

 has been seriously questioned by some, by others denied. 



The rocks under consideration are scattered through the central part of the 

 State within the eastern half of the Piedmont plateau— topographically the median 

 one of three zones into which the middle Atlantic slope has been divided. 



The granites of this region comprise four petrographically distinct types: Binary 

 granite. Granitite, Hornblende granite and Allanite-epidote granite. 



*Bul. U. S. Geol. Sur., No. 52. 1889. 



