Correspondence. 63 
St. and forming the valley between the hights of Columbia University 
and those upon which the convent of the Sacred Heart is situated is 
frequently brought forward in support of this view. In this rock at 
the contact with pegmatyte dikes very good specimens of foetid quartz, 
phlogopite, white pyroxene and brown tourmaline are obtained. The 
Inwood limestone, called by Merrill Cambro-Silurian, is exposed at 
7th Ave. and 155th St-, and at the typical locality Inwood, immedi- 
ately north of Fort George (Amsterdam Ave. and 119th St.) 
Beneath this limestone is the Fordham gneiss which is thought tc 
be of igneous origin and of Archean age. An unquestioned exposure 
of this rock is not known upon Manhattan island ; but north of the 
island are abundant exposures. 
Above the Manhattan schist is the Triassic sandstone which is well 
shown on the western side of the Hudson. The base of this sandstone 
a conglomerate, is well exposed in a cut made by the West Shore R. R. 
at Stony Point- The sandstone is penetrated by the Palisade sheet of 
diabase. The sandstone both above and below the diabase is baked 
into a hornfels in which the characteristic metamorphic minerals are 
usually present. In places this becomes a tourmaline hornfels from the 
great development of that mineral. The effect of the cool walls upon 
the intruded igneous mass is very evident as one follows the sheet from 
its lower to its upper contact on the sandstone. The middle of the 
sheet is very coarsely crystalline but becomes quite fine grained as the 
wall is approached. Mr. A. L. T. Quereau has lately given the re- 
sults of his study on the subject in the School of Mines Quarterly, 
Vol. XXIII. This diabase sheet with its columnar jointing forms 
the well known Palisade on the west bank of the Hudson river. The 
lower contact is well shown at the 130th St. ferry. Here is an excel- 
lent place to note the gradual change in the texture of the diabase as 
the Palisades are ascended. The upper contact is shown at the tunnel 
of the West Shore R. R. at 34th St. ferry. Besides the intruded sheet 
there are above a, farther west in New Jersey three surface flows ; 
for the upper surface of each is amygdaloidal. The first or lowest 
flow is well shown at Feltyville. At the Great Notch on the Green- 
wood Lake division of the Erie R. R. is an excellent place for the 
collection of minerals. These occur between two of these surface 
flows in the interstratified baked shale. Here are found quartz, cal- 
cite, analcite, heulandite, stilbite, prehnite, natrolite, datolite, pectol- 
ite, chabazite and other minerals. Faults are quite numerous in the 
Triassic, one of which is exposed at Kingspoint near Hoboken. 
Another interesting rock which the classes always visit is serpen- 
tine. It is well exposed in Hoboken at Castle Point. The problem 
of its origin is still debated, but is thought by most to be an altered 
peridotyte or pyroxenyte, since the presence of pyroxene, amphibole 
and olivine has been determined. The processes of serpentinization 
are however nearly complete. This is the home of various magnesian 
minerals as brucitc, nemolite and magnesitc. Chromite is also 
abundant. 
