134 PRIMORDIAL—WAPPINGER LIMESTONES. 
at a right angle to the strike. This line runs along the 
southern margin of the driving-park, continues east 
through the fields parallel to Hooker avenue till it 
meets the Hudson river shale in the meadows of Casper 
creek. Immediately north of the driving-park, in 
fields on the north side of Hooker avenue (locality O, ; 
plate I), in place of limestone there are hills of Hudson 
river shale, and this same rock crops out along Hooker 
avenue to the east all the way to Casper creek and be- 
yond. From this line these shales continue unbroken 
many miles to the north beyond Hyde Park. Along 
this northeasterly and southwesterly line, the interven- 
ing space between the outcrops of shale on the north 
and of limestones on the south consists of a mostly level 
strip of a drift having bogs and springs along its 
southern edge. Evidently this is a line of fault, across 
the strike, between the Hudson river shales on the 
north and the Potsdam, with its associated Calciferous 
and Trenton, on the south. This trough, cut through 
the hilly ridges, is a conspicuous feature in the local 
landscape. It is made available for traffic by the avenue 
that passes through it obliquely into the city ; it fur- 
nishes the long and smooth plat (mostly underlain by 
the shales) which constitutes the driving-park. 
The stratigraphic relations of the western margin of 
the belt of Potsdam have proved far less obvious 
than those of its northern extremity. But after a care- 
ful, detailed inspection of the ground, a correct solution, 
as I believe, has at last been reached. During the 
earlier examinations, Calciferous or Trenton strata were 
naturally looked for, in intervening folds, between the 
Potsdam and the Hudson river shales to the west. 
The western hill of conglomerate above mentioned (hill 
B) and its southern extensions, were at first taken to be 
the Trenton coralline conglomerate which occurs abund- 
antly in the vicinity. But on further investigation, two 
is 
