I queried Ivan "Resigned-from-BP-to-get-a-life" Myjer about something I have
observed regarding NYC sidewalks.  In the Tribeca/Soho areas (all commercial
buildings, 1860's-1900's) the surviving stone sidewalks are all granite (or
just igneous?), with long spans (no doubt to cover the underground vaults).
In residential/brownstone areas (1870's-1910's), surviving sidewalks are
smaller flags (because they have no need for the vault space, but they are
all SLATE OR BLUESTONE.

I had originally posited that slate or bluestone would fail at such a long
span, but Ivan's message (given below in full), says ix-nay.

Does another subscriber have another theory?

Signed,  Christopher "Once-had-a-life" Gray

BEGIN IVAN
Subj:    Re: granite vs. bluestone flagstones
Date:   6/28/99 8:41:50 AM Eastern Daylight Time
From:   [log in to unmask] (Ivan Myjer)
Reply-to:       [log in to unmask]
To:     [log in to unmask]

Granite and bluestone at 16" to 20" the thickness used for large flagstones
are equally strong in compression. I am not sure about tensile strength but
I would think that bluestone holds up as well.
Over the years I have been asked several times if I knew where to get 12
foot long slabs to replace bluestone pavers in NYC. I do not know if they
were over vaults. I suspect that at that length they were spanning vaults.

I suspect that the time of original construction is a factor. I do not know
when bluestone first appears as a flagstone. Merrill, writing in 1910 says
that in 1880 there were over 150 bluestone quarries in the area west of the
Hudson River below Albany.

Due to innovations is quarrying, large pieces of granite for construction
became available in the 1820's
Currently, bluestone is quarried on the New York side of the Delaware
River. Large slabs are no longer available due to the limitations of the
cutting equipment, not the stone beds.

Ivan
END IVAN