faces
are as universal as the boulders so frequently found near them. In Dr.
Gale's examination the record of grooved surfaces challenges our
admiration from the persistency of their occurrence, for whenever the
rock was uncovered, exposed, in all probability, for centuries to rain
and frost and heat, these extraordinary rulings remain, altered, of
course, partially erased, but never obliterated. A few examples of
these early notes are of instructive interest. It may be premised that
the gneissoid rocks of New York are not the best arranged or composed
mineral aggregate for the preservation of these grooves. The
decomposable character of the mica and feldspar, their considerable
permeability to water, and the varying hardness and denseness of the
rock furnish a poorer surface than more homogeneous rocks, as
sandstones, limestones, and fine-grained granites, so that the grooves,
scratches, and striae over the gneisses and mica-schists oi Manhattan Island undergo a gradual blurring, and are best preserved where the rock is more siliceous and more dense.
At
70th Street and 10th Avenue Dr. Gale records abundant drift grooves
and scratches, in direction N. 35° W.; again, at 77th Street (same
avenue), " numerous, and extending almost everywhere in favorable
situations into the river even below the lowest tide-water marks, and
again to the highest elevations on the island." Again, on the
Blooming-dale Road, between Bloomingdale village and Manhattan-ville,
where the rock rose seventy feet above the Hudson, the whole surface,
over four thousand square feet, was scored with grooves from half an
inch to an inch in depth, from one to six inches in width, and from
forty to sixty feet in length. North of this were many more, some
reaching three inches in depth, so that, in fact, for thousands of feet
over the rock at the water's edge and on the highest points he records
the omnipresent grooves.
Grooves
are yet traceable on the rock shoulders above Mofningside Park, south
of 118th and 120th Streets, but strong channelings are reported from
163d Street and St. Nicholas Avenue.
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