Marble: the stony truth...

["Dr. Tim Palmer" <tjp@zzzzzzzzzz>]

I'm not familiar with all the North American marbles, but offer this as a
suggestion.

Limestones were built up as a series of beds, one on top of the other, each
successively deposited on the original sea-floor.  Often the beds vary
slightly in composition (more clay in some: less in others).  Sometimes the
beds are separated by thin concentrations of clay (partings; bedding
planes).  Furthermore, in some cases dissolution of the limestone beds took
place along the bedding planes as a consequence or pressure from the weight
of the overlying rock.  Under this circumstance, the insoluble clays etc
that were formerly dispersed throughout the limestone bed, become
concentrated into layers.

If rocks that underwent some or all of these processes then become turned
into low grade marble by heat and pressure, then the concentrations of
insolubles would have remained as veins, still parallel to the original
bedding of the limestone.  With higher pressures and temperatures, I
suspect that the veins would start to take up other orientations as the
calcite crystals grew around them and forced them into different orientations.

Incidentally, UK television is starting to advertise a programme about
Michaelangelo (I'm sure it will appear in the US and elsewhere soon).  The
programme makers had a long consult about marble and its formation early
on, and spent time in the Carrara quarries.  They expressed the intention
of saying something about this in the finished programme.  It might be
worth looking out for.

Tim