My thing of kosher salt, at least, claims to be pure “salt”, no additives, so I assume they mean pure NaCl.
Too bad it vectors the myth people were paid in salt.
Wow, very interesting! Quite a process and good input from so many! I hope my slow reply wasn’t…in-salt-ing! 
But, thanks, y’all!
Three reasons why calcium chloride is preferred over sodium chloride (though ultimately the decison will most likely based on the price of the available chemicals):
[ol]
[li]Robby pointed out that calcium chloride is CaCl2 (don’t know how to do subscripts in the editor) whereas sodium chloride is NaCl. Molal freezing point depression is directly related to the ionic concentration of the solution and calcium chloride will give you 1.5 times the ionic strength at the same molar concentration.[/li][li]Sodium chloride is much more corrosive to concrete and steel (in autos) than calcium chloride (or magnesium chloride for that matter).[/li][li]The dissolution of calcium chloride in water is more exothermic than for sodium chloride.[/li][/ol]
CaCl[sub]2[/sub]
Surround the subscripted text with the tags [noparse][sub][/noparse] and [noparse][/sub][/noparse].
(And to see any fancy editing on text, you can always hit the Quote button, look at the way it was originally posted, then cancel out of the quote window.)
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Good articles on the mining, itself, are at Metropolitan Mining: Institutional and Scale Effects on the Salt Mines of Detroit and The Encyclopedia of Cleveland History: ROCK SALT.
Everything I know about salt I found out watching the salt episode of How Stuff Works, print recap here.
Here’s a Cleveland-area salt mine from a few years back: http://1.bp.blogspot.com/-uwzPpSB7VUM/TWK1nZBUcXI/AAAAAAAAAAc/TD9ab9C0ByI/s1600/salt4.jpg
We tried to arrange a tour for the Cub Scouts recently, but alas, they don’t do that anymore.