Baltimore and the Ohio River. 221 



cd and furnish a tolerable, coarse roofing slate. The arenaceous va- 

 rieties are often crossed with veins of quartz, and are occasionally 

 colored with chlorite. All the varieties vary much in color, the most 

 prominent are black, blue, green, chocolate color and red. And all 

 the varieties of color and texture may be seen intermingled with each 

 other for a distance of thirty miles. The glazed varieties are closely 

 allied to what was once called par excellence, " primitive argillite." 

 The lalcose glazing of this parti-colored rock is noticed by Prof. 

 Eaton,* who quotes Dr. Higgins for an ingenious explanation of the 

 different colors. Being most probably caused, says Dr. H., by the 

 combination of magnesia .with iron in different degrees of oxidation 

 which gives blue, purple, and red compounds, the green being pro- 

 duced by chlorite. This would suit our slates very well, for nearly 

 all are more or less talcose, and frequently chloritic. The author's 

 description of graywacke slate, in another part of the same volume, 

 applies admirably to our entire graywacke formation. u This rock 

 takes on the greatest variety of character of any rock in our district. 

 It is coarse and harsh, soft and smooth, fissile and compact, brittle 

 and strong, gray, blue, green and red. Notwithstanding these vari- 

 eties, there is a peculiarity in the rock by which we recognise it af- 

 ter seeing it once."f Succeeding the transition graywacke, we next 

 meet the genuine transition limestone of Frederick Co. Md. ; it first 

 appears on the right bank of the Monocacy, a few miles east of Fred- 

 erick city. From thence it continues westward, underlays that city, 

 and only disappears where the slate and conglomerate of the Catoc- 

 tin ridge have been forced into view. Between the limestone of 

 Frederick valley and the quartzose grit of the Catoctin ridge, lying 

 below the former and above the latter, there is interposed a bed of 

 calcareous breccia, the well known Potomac marble, which has fur- 

 nished, though from another locality, the beautiful pillars of the cap- 

 itol. I have not visited the quarry, (some twenty miles south of the 

 section,) where these were procured, but have reason to believe that 

 the formation is continuous from that point, to one a little west of 

 Frederick city. That it continues still farther north is proved by its 

 having been found by Prof. Ducatel, and Messrs. Tyson and Alex- 

 ander, in the vicinity of Mechanics town, a distance perhaps of about 

 fifteen miles in a right line. At this last place it exhibited the ap- 

 pearance of terminating, so that probably it would be best considered 

 as an accidental bed, formed from the ruins of some previous calca- 



• Canal Survey, p. 64. t Mem, p. 88. 



