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196 GEOLOGY OF NEW YORK CITY
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smaller specimens that crowd in exhaustless trains upon the footsteps of their larger companions.
In
short, we gather the irrefragable testimony, wherever we look for it,
through our Northern States, through Europe and Asia, and even along
the western coast of South America, that some immense force has been
exercised in time past, not only to dislocate and shatter the rocky
barriers which opposed it, but also to carry them in its southward
movement far removed from their place of origin. Further, let it be
remarked that, though one class of these erratics is composed of
angular and unworn stones, another yields boulders that have undergone
severe attrition, and along their larger axes are striated and
polished; bearing in mind, moreover, that the direction of their
transit coincides with that of the furrows and flutings in the same
region, we may strictly conclude 4hat they are a feature also of the
same excessive and gigantic system of erosion.
But
there is a group of deposits of a yet broader and more significant
character in its general relations than the foregoing. Over Scotland,
England, Ireland, Scandinavia, Denmark, Central Europe, Switzerland,
Prussia, France, Spain, and in North and South America, in short,
wherever we discover boulders and grooved surfaces, we find a deep and
characteristic deposit, not the work of alluvial formations or recent
detritus, for it underlies these, but the record of a vast
disintegration which has covered the land with sheets of gravel, clay,
and sand, all intermixed with stones and boulders, variously combined
in their order of succession, and ranging in depth to over 300 feet.
These immense beds furnish gravel for roads and ballast, sand for
glass making and mortars, and clay for brick; their included stones and
fragments are scored and embroidered with fine and interlacing striae,
and they cover the furrowed surfaces of either hemisphere for miles.
They represent the accumulated wear and tear of continents, under some
extraordinary agent of erosion and denudation, whose teeth have
resistlessly ground upon the
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