320 FUCOIDES IN THE COAL FORMATIONS. 



geologist is better acquainted with the distribution of the measures in the whole extent of 

 Pennsylvania, that considering the anomaly of the presence of the Chemung in that part 

 of the State, I began a stratigraphical survey of that country, disregarding every kind of 

 palaxmtological evidence. 



Beginning at Homewood Station in Beaver County, the Millstone grit is there exposed 

 along Beaver River with a thickness of one hundred and sixty feet, its base resting on a bed 

 of ball and clay iron ore, soft black shales, with pebbles of carbonate of iron, thin layers 

 of coal, &c. Higher up, at Homewood Furnace, and at the mouth of Coneconessing Creek, 

 the Millstone grit is still one hundred and ten feet thick, and is underlaid by the same kind 

 of shales and ball iron ore. Up the Coneconessing the stream flows between banks of the 

 Millstone grit, which slowly decreases in thickness. At the mouth of Smalley's Run, six miles 

 above, these measures are only sixty feet thick, and the Subcarboniferous strata exposed 

 there show the same nature and distribution as at Homewood Station. The thinning con- 

 tinues at about the same rate to the mouth of Slippery Rock Creek, where banks of hard 

 Conglomerate, forty feet thick, descend to nearly the level of the river. Six to eight feet 

 of Subcarboniferous measures are exposed here at low water. From here, in ascending 

 Slippery Rock Creek, the decrease in the thickness of the Millstone grit becomes more 

 rapid and irregular ; these strata changing here and there into shaly sandstone, five to six 

 feet thick, then disappearing entirely, to be seen in place again a little higher up in the 

 creek. The last appearance of the Millstone grit is just below the lower mill at Wurtem- 

 berg, where the sandstone, still hard and gritty, is six feet thick, and the black shales and 

 clay iron ore are exposed under it six to eight feet thick ; there it definitely loses itself 

 in a thin bed of soft shaly sandstone, wedging into the top of that clay iron ore which in 

 the section is marked as under the bed of coal. From Wurtemberg these strata, preserving 

 the same character and horizon, continue along the creek, without any trace of sandstone, 

 to six miles above, where the Millstone grit reappears in the same manner and at the same 

 horizon as it is seen passing away at Wurtemberg, and rapidly increasing in thickness. 

 At Seceder's Bridge, nine miles above Wurtemberg, it is already one hundred and ten feet 

 thick, underlaid by forty-nine feet of Subcarboniferous measures. 



From these observations it follows : 



1st. That the whole thickness of the strata marked in the section of the hill opposite 

 Wurtemberg, including the lower bed of coal and the limestone with Fucoidal remains 

 {(Juulerpites marginatus), belongs to the Carboniferous formations. 



2d. That, from Homewood Station to Seceder's Bridge, a distance which, in a straight 

 line, is not more than fifteen miles, there is in the Millstone grit formation a wide, nearly 

 abrupt gap, about five miles broad, where the Carboniferous measures, immediately over- 

 laying the shales of the Subcarboniferous, are mostly marked with remains of marine 



