58 



OUTLINES OF GEOLOGY. 



it at Sandusky, and did not lose sight of it on the eastern side 

 of the Mississippi. To the north, it extends to Michilimac- 

 inac, and far to the south in Kentucky. In this great forma- 

 tion are situated the lead mines of Illinois and Missouri. On 

 the eastern side of the Allegany ridge, it is comparatively 

 rare. I however found it in its true position in relation to 

 the coal measures, at Cumberland, in Maryland, and form- 

 ing a parallel mountain range, known from its rounded peaks, 

 under the name of Knobley. It is also found at Ticonderoga, 

 in the state of New-York. 



139. The carboniferous, usually called the old red sand- 

 stone group, derives the latter name from a rock strongly 

 resembling the true sand-stone of the triasic group. This 

 rock is evidently of mechanical origin, and is composed prin- 

 cipally of quartz, which is mingled with mica and felspar. It 

 also contains fragments of the rocks of the submedial and 

 inferior order, and thus runs into a conglomerate. The mode 

 of union, however, appears to be rather by aggregatian than 

 by a cement. If the red sand-stone formation of New-Jer- 

 sey do not belong to this group, but to the triasic, red shale 

 is not one of its rocks. Its lower beds pass into grauwacke, 

 a rock which differs from conglomerate, in being made up of 

 angular fragments, held together by aggregation. 



140. The upper beds of the carboniferous sand-stone al- 

 ternate with the lower beds of the foregoing limestone ; and 

 its lower beds with the upper beds of the limestone of the 

 submedial group. The layers of sandstone are often divided 

 by beds of clay, either soft or indurated. 



141. Organic remains are very rare in this group, being 

 wholly wanting in its upper beds, but appearing in the gran- 

 wacke. In this country, thin seams of coal occur frequently 

 in this formation, and vegetable impressions may therefore be 

 looked for. 



142. The red sand-stone of this group is very barren of 



