THE CHROMITE DEPOSITS OF THE STATE LINE SERPENTINES.^ 



By Samuel G. Gordon. 



The chromite deposits described below occur in a belt of ser- 

 pentine situated on the Pennsylvania-Maiyland line, in Lancaster 

 and Chester counties, Pennsylvania, and Cecil County, Maryland, 

 In the last century the district was an important producer 

 of chromite, and during the recent war attempts were made to re- 

 open the mines, but with little success. Aside from economic 

 considerations, the deposits are of considerable scientific interest, 

 as the data regarding chromite deposits are rather meager, beyond 

 the fact that they appear to be true magmatic segregations without 

 the aid of ''mineralizers." 



General Geology. 



The geology of the district has been described by Rogers^, 

 Chester^, Frazer*, and Bascom^. The serpentines of the belt 

 represent hydrothermal-metamorphosed peridotites and pyroxen- 

 ites, part of an igneous complex forming a large batholithic mass 

 overlain on the north by mica gneisses. The following interesting 

 petrographic succession is exhibited in passing northwestward 

 across the batholith-biotite-grandiorite, hornblende-biotite-grano- 

 diorite, quartz-biotite-hornblende-gabbro, quartz-hornblende-gab- 

 bro, hornblende-norite and quartz-norite, norite, pyroxenite, and 

 peridotite. The main types are considered by F. Bascom to re- 

 present differentiation of the magma before intrusion, while their 

 gradations are thought to represent subsequent differentiation. 



1 The writer is indebted to Dr. Edgar T. Wherry for a critical review 

 of the manuscript, and to Mr. F. Lynwood Garrison for various courtesies 

 extended to him during his visit to the district in April and August, 1920. The 

 albitites were described in a previous article in these proceedings, and an account 

 of the minerals of the district will appear in The American Mineralogist. 



^ Henry D. Rogers, Geology of Pennsylvania, 1858 



3 Frederick D. Chester, Ann. Rep. Second Geol. Surv. Penna. 1S87, 93-105, 

 1889. 



* Persifor Frazer, Jr., Second Geol. Surv. Penna. Reports C3 and C4, 1880 and 

 1883, Geology of Lancaster County and Chester County. 



6F. Bascom, Cecil County; Maryland Geol. Surv. 1902, 83-148. 



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