I put the sidewalk question (why it's bluestone in residential districts,
granite in commercial districts) to Sidney Horenstein, geologist at the
American Museum of Natural History [although he answers his phone "Hello,
invertebrates"] and authority on NYC stone.

He says that, notwithstanding test results, strength was the determining
factor for choice.  19th century B-P'ers knew from trial and error that
bluestone would fail under wagon wheels and the long span over the vaults.
He says that he has never seen any period account of this practice, but this
is how he explains what appears to have been a consistent practice.  [Ralph:
sorry - I forgot to ask him about "killer" wisteria -  maybe Henry Stern?]
Sid does note that of all the granite sidewalks, he has never seen anyone
using the white granites - conceivably because they would soil more quickly,
but more probably [he thinks] because they are intrinsically weaker.

Sid observes that bluestone was cheaper to extract, and closer to NY, and
thus [he surmises] cheaper.  That might account for bluestone's default
position in residential work, or, perhaps, bluestone was considered "more
fitting" for domestic work.

Sid finally observes that, although 19th century stone-persons would have
referred to the material we see in the commercial districts as "granite", to
a geologist the slabs with the heavy figuring and wide bands are actually
metamorphically altered granite, generally gneiss.  True granite itself is
homogeneous - just like this list!

Submitted, Christopher Gray