300 C. R. KEYES — MARYLAND GRANITES AND THEIR ORIGIN. 



Sykesville, < ruilford, < rarretl park, Woodstock, [lchester, Ellicotl City and 

 Dorseys Run. 



Tin ir mineralogic Composition. — Microscopically the rocks under con- 

 sideration are composed largely of quartz, feldspar and mica, with acces- 

 sory plagioclase, microcline, magnetite, apatite zircon, epidote, allanite, 

 hornblende, sphene and often some other minerals. 



Four Till"* represented. — Aside from their structural phases, the Mary- 

 land granites comprise four types : binary granite, granitite, hornblende 

 granite and allanite-epidote granite. Each has been fully described in 

 another place. 



Origin of the Granites. 



Two Theories advanced. — Regarding the origin of granites in general two 

 leading theories have been advanced. One considers a granite as the 

 last stage in the metamorphic change of mechanical sediments. With 

 the other a granitic mass is regarded as the product of the gradual 

 cooling of an acidic molten magma, and it is commonly supposed that 

 the cooling takes place under pressure. 



Recently great stress has been laid on the metamorphosing influences 

 of orographic movements in disguising the original character of rocks, 

 making eruptives more and more like sedimentary deposits, and clastic 

 beds more and more like massives. But without entering into a discus- 

 sion of the general subject, it is intended here to merely set forth some 

 of the proofs that point to the eruptive origin of certain of the Maryland 

 granites. That these particular rocks are really eruptive in character 

 has been seriously questioned by some investigators, while by others the 

 eruptive character is denied. 



Localities favorable for Observations. — Perhaps the most favorable places 

 for observing phenomena hearing upon the origin of certain of the Mary- 

 land granites are at Dorseys Run station, Woodstock, and Sykesville. all 

 on the main line of the Baltimore and Ohio railroad a few miles west of 

 the city of Baltimore. 



At the first-named locality an excellent section has been exposed by a 

 recent railway cutting. Here a dark-colored gneiss is found penetrated 

 by a light-colored granitite. Huge blocks of contorted gneiss, often 10 

 to 20 feet across, and numerous smaller angular fragments are embedded 

 in the massive granite. Souk.' of the gneiss blocks are twisted and bent, 

 to all appearances through movement when the granite was in a viscous 

 state. Very light-colored granite dikes also cut in various directions. 

 Chemical analyses show the dikes to he very much more acidic than 

 either the gneiss or the massive granite. 



