60 KIMBERLITE FROM THE UNITED STATES
a
lawn and a street. His description was founded upon observations made
and specimens collected, about 1887, by Professor Oren Root, who found
it to be associated with shales and limestone belonging to the Onondaga
salt-group, and considered it, for reasons which he gives, to be
intrusive in them.
In
a later paper' Professor Williams adds some interesting details. In
cutting some deep sewers, ' two admirable sections ' had been exposed.
In these it was seen that the serpentine formed a dyke, cutting
perpendicularly across the nearly horizontal strata of limestone,
forcing its way in places between them, and, in one of the exposures,
considerably disturbing those in its immediate vicinity. Much of the
serpentine was full of angular fragments of other rocks embedded in it,
sometimes at least one-third of the face of a block being occupied by
fragments. Most of these, but by no means all, are bits of the adjacent
limestone. One, for instance, is a large fragment of black shale
(probably Utica shale), which here is over 1,000 feet below the
surface. Another is a fragment of an acid crystalline rock, granite or
gneiss, which must lie over 2,000 feet below the surface. Granitic and
syenitic fragments were also mentioned by the earliest observers. All
of the included limestone fragments show signs of contact metamorphism,
and the new minerals, thus produced, have a zonal arrangement.
Professor
Williams is of opinion that the eruptive rock, which is now represented
by the serpentine, must have differed much from its present composition
as represented in analyses furnished by Professor James Ball, which he
quoted in the earlier memoir. The specimen consisted of carbonate of
lime 34*77, carbonate of magnesia 2'73, and serpentine 62-5. The
composition of the last was as follows :—
1 hull. (tool. Soc. Amer. i. 1890, p. 533.
