VaJti'ozoic JUdiii'ij of J^rinixi/I i'<iit/((. — i'hiyjioh:. 155 
Prof. White adds : "The Selinsgrove upper sandstone is one of those 
intercalated beds whicli liere makes its appearance in the middle of the 
Hamilton group and is so thick and massive as to change entirely the 
character of the topography, for instead of a wide level valley as in 
the north, it is now found making a high ridge along the strike of this 
sandstone. There is not an inch of this sandstone represented in the 
bed on Fishing creek. (Fishing creek drains nearly all of Columbia 
Co. north of the Susquehana and is therefore to the northeast of North- 
umberland Co.)" 
Again we tind in Mr. Piatt's report on Blair Co. the following 
evidence. (T^ p. 31) : 
"In Blair Co. the whole of the Hamilton formation is composed of 
dark shale with calcareous layers." 
And further Prof. Lesley adds a note in the report of Rye 
township (Fg p. 310) to this effect : 
"The outcrop of the Hamilton sandstone crosses the Susquehanna 
river eastward from Perry Co., and runs through Dauphin, Lebanon and 
Schuylkill counties," that is northeastward. 
It thus appears that the Hamilton sandstone gradually thins 
awa}' from a point near or on the south line of Perry Co., to the 
northeast, north and northwest, in which directions alone it can be 
traced. In thus thinning out it splits and becomes a lower and 
an upper sheet of which the latter is the more persistent, extending 
even into Northumloerland, while both underlie much of Hunting- 
don Co. Its color is usually' gray or whitish, seldom yellow and 
never red, and it is usually not very hard. It is not conglomeratic. 
At the south line of Perry Co. it stands vertical and is even 
somewhat overthrown as ma}' be seen upon the Susquehanna a few- 
miles north of Harrisburg. When this is tlie case it forms ver}' 
rough and almost mountain territory. Its southernmost outcrop 
is known as Little mountain and is cut b}' the Pennsylvania Rail- 
way near the bridge across the Susquehanna at Rockville. 
How much farther it may hare extended to the southeast is of 
course unknown. The contortion and erosion which the countr}' 
has suffered since its deposition have utterly destroyed all trace of 
it. Moreover along the line of its last southern outcrop in Little 
mountain lies apparently the edge of a great overthrust plain which 
has brought the Onondaga red shale against the Hamilton sand- 
stone, cutting out, that is covering up, the intervening strata 
though a thousand feet in thickness. These only reappear to the 
