Wow, you're all getting into a lot more detail than I'd think anyone would
be interested in knowing about.  But yes, slates are compressed and heated
shales (or better yet clastic rocks...geologists see a lot of gray area
since all we're really talking about in these definitions is grain size and
what has happened since the muck was deposited).  Imagine a few hundred feet
of nicely horizontal beds of mud and silt and then picture a continent
slamming into those beds.  There's a great deal of heat and the constituent
minerals begin to recrystallize.  Clays become fine-grained micas.  However,
these new platy minerals orient themselves at a high angle to the direction
of the compression.  This is what produces the cleavage or rift in a slate.
If you look closely at some slates you will often see a faint banding across
the split face.  This is the original bedding orientation that is now at an
angle to the rift.  It's best observed when there are sandier layers
present.  If you get a little hotter and squishier the slates will first
become something called a phyllite, then a schist, then finally a gooey
mess.  

And if you think cleavage, hardness, graywacke and schist are funny, you can
imagine the giggling in mineralogy class when the professor first introduced
the mineral "cummingtonite".  I schist you not.

John

-----Original Message-----
From: The listserv where the buildings do the talking
[mailto:[log in to unmask]] On Behalf Of Gabriel
Orgrease
Sent: Tuesday, November 11, 2008 6:12 PM
To: [log in to unmask]
Subject: Re: [BP] New (maybe) Thread

John Walsh wrote:
> Slates are recrystallized rocks
due to compression?

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