136 PRIMORDIAL—WAPPINGER LIMESTONES. 
as far as the turn in the Ferris road, beyond which the 
demarcation is produced by a different but no less strik- 
ing plan; for south of this point the limestone remains 
a high conspicuous ridge, abruptly steep on the western 
side, while the shales, mostly covered by drift, form a 
level plain at its base. 
These facts make it evident that there is also a line of 
fault between the Potsdam and the Hudson river shales 
at the western margin of the limestone belt, more or less 
parallel to the strike. The general direction of this line 
of fault is about N. 40° E. The Potsdam strata near the 
line of contact are generally deflected in such a way as 
to have less easting in the strike, which is in such places 
from N. 4° BE. to N. 11° KE. Thus at the extreme north 
end of the main fossiliferous hill (A) the strike varies 
between the limits just given, while the dip becomes as 
low as 20°. At the hill of calcareous quartzyte in the 
third field south of the driving park (hill F) the strike 
- is in some places N. 11° E., and the dip 35°. 
On the other hand, the Hudson river shales incline 
to acquire more easting in the strike, in the vicinity of 
the plane of contact. In such cases they have quite fre- 
quently a strike of N. 46° E. (their general strike in the 
vicinity being about N. 31° E). They also, in such posi- 
tions, are often found inclined at a very high angle. 
These phenomena seem to indicate considerable friction 
by a lateral motion in a northeasterly and southwesterly 
direction, as well as in the upward motion, at the time 
of the uplift of the Potsdam. This fault is the more in- 
teresting, because it is evidently related to the great 
fault described by Sir William Logan and Prof. 
James Hall as extending from Quebec to the Hudson 
river near Rhinebeck. If, as has been suggested by 
Prof. J. D. Dana, this should more properly be regarded 
as aseries of more or less parallel faults, the present 
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