ABSENCE OF ERUrXIVES FROM WESTERN AREA. 307 



more pronounced when seen under the microscope, as is more fully described 

 and figured in the accompanying paper by Mr. Keyes. The cement, which 

 is sometimes silicious (chalcedouic), sometimes kaolinitic, shows no recrys- 

 tallization, nor even any enlargement of the clastic quartz grains. There 

 are no new minerals developed, so that the rock cauuot be regarded as hav- 

 ing suffered any appreciable amount of metamorphism. 



Marble. — The limestone, which is abundant in small thin lenses in the 

 sericite schists north of the Baltimore and Ohio main stem, is intermediate 

 between the blue uucrystalline rock of the Frederick valley and the coarse 

 saccharoidal dolomites of Baltimore county. It is a marble of extremely 

 fine, even grain— almost cryptocrystalline in texture — and of unusual hard- 

 ness. Its color is sometimes snowy white, as at the Westminster quarries,'=^ 

 but more often streaked and variegated with black, gray, or red. This marble 

 does not contain its argillaceous impurities in the form of crystallized sili- 

 cates, as is the case with the more eastern limestones, but as narrow bands 

 of sericite or chlorite schist. It is in connection with this compact marble 

 that the copper (Liberty and New London) and lead (Union Bridge) de- 

 posits of Frederick and Carroll counties, which form contact-bodies against 

 the adjoining schist, occur. 



Eruptive JRocks. — The western area presents a marked contrast to the 

 eastern and more crystalline area in the almost entire absence from it of 

 eruptive material. What there is belongs either to the Mesozoic diabase or 

 to serpentine whose origin is still in some doubt. 



ROCKS OF THE EASTERN OR HOLOCR YST ALLINE AREA. 



Sedimentary Series : Gneiss. — The rocks of the western area are for the 

 most part devoid of feldspar, or, if this mineral is present, it is clearly of 

 detri^al origin. The rocks of the eastern area, on the other hand, are highly 

 feldspathic, and this mineral has crystallized in its present position. The 

 sedimentary rocks of the eastern area have been penetrated by vast quanti- 

 ties of eruptive material, often of a composition similar to that of the beds 

 through which it breaks. Both of these have subsequently been subjected 

 to such intense dynamic metamorphism that it is now not easy to dis- 

 tingush those of different origin. Several areas of what were at first re- 

 garded as typical sedimentary gneisses have been found by their inclusions, 

 contact action and apophyses to be really eruptive granites, in which a folia- 

 tion and even banded structure was subsequently developed by pressure. 

 Other masses, now considered as representative gneisses, may hereafter also 

 be referred to eruptive types. With our present knowledge we may, how- 



*In slaty band? traversing; the«e quarries, which lie very near the eastern border of the semi- 

 crystalline area, Professor P. R. Uhler tells me he found fossils— well-characterized shells— about 

 1880. The specimens were, however, never described or identified, and were lost at the New Orleans 

 exhibition. The lucky find has not yet been repeated. 



