John Walsh wrote:
> Bluestone is a dark-colored and indurate "dirty" sandstone.  It's rift is
> generally somewhat thick resulting in inch-scale flags.  Slate is a
> finer-grained and lightly metamorphosed mudstone/siltstone.  The rift is
> usually more penetrative resulting in thinner slabs and there usually is a
> secondary potentially weak plane at an angle to the first.  That's a
> geologists view.  Architecturally, they look different in texture though
> often similar in color.  Engineeringally (yes, that's a word), bluestone is
> usually harder and stronger.
>   
thnk u John

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