220 Geology of the Country between 



possible manner. It is no easy task to reduce this confusion to sys- 

 tematic arrangement, and there seems abundant room for diversity of 

 opinion, with but little danger of any one proving his opponents in the 

 wrong or himself in the right. It is indeed difficult to convey a cor- 

 rect idea of the primitive strata along this part of our section. Slaty 

 and crystalline granite appear to predominate, mixed with slates and 

 a rock approaching hornblende rock in external character. The 

 granite may be well seen in the neighborhood of Ellicott's mills, 

 where there are extensive quarries that furnish vast quantities for the 

 Baltimore market. Succeeding the primitive rocks, next appear 

 transition slates and sandstones, exhibiting the usual Protaean varie- 

 ties of the transition graywacke formation. I use this last term in 

 the general sense adopted by Humboldt, who designates by it* " ev- 

 ery conglomerate, sandstone and fragmentary or arenaceous rock of 

 transition formation that is anterior to the red sandstone and coal for- 

 mation," with this addition, that I would also include within the same 

 definition all those transition slates that we find interstratified with the 

 above conglomerates and sandstones, and which must have been of 

 nearly contemporaneous origin. This would include the argillite in 

 all its varieties, the old red sandstone, the millstone grit and the gray- 

 wacke, and graywacke slate of Prof; Eaton, as members of the same 

 formation. There would at first sight seem but little analogy be- 

 tween the soft roof slate of commerce, and the harsh quartzose con- 

 glomerate that is quarried for millstones. But the wide difference 

 between these disappears when we find, first, this conglomerate chang- 

 ing its character and passing gradually into finer sandstone, next, the 

 sandstone becoming more slaty and alternating with beds of genuine 

 argillite. Prof. Eaton has observed that Europeans do not under- 

 stand their graywackes, and I might safely add that no one can well 

 understand the graywackes of this country without visiting the moun- 

 tains of Pennsylvania and Virginia. This formation, which is colored 

 brown on the section, commences on the west of the primitive rocks, 

 that are colored red, and continues uninterruptedly to the Monocacy, 

 including the elevated ground known as Parr's ridge. Throughout 

 this whole distance, its peculiar variable characters are strikingly ex- 

 hibited. The slaty varieties often appear somewhat talcose, often 

 shining as if varnished, at one time friable and rapidly disintegrating, 

 and again firm and compact. At one place quarries have been open- 



Superposition of Rocks, p. 201, 



