[Cob] Silverfish, Tornadoes, and Humidity

[Amanda Peck]

You don't want round rock for the same reason you don't want round 
sand--just a difference in scale.

Sara Andrews, in her mystery Fault Line has some memorable passages on the 
subject.  In the book a moderate earthquake in Salt Lake City hit the older 
parts of the city hard because the sand had been from the Great Salt Lake, 
washed and worn smooth and round over the years.  What's classed as a 
micro-quake would do immense damage in Boston because the sand, not the 
mortar parts is all that's holding the aged mortar in place. (not sure if 
pure lime, already turning to limestone would have the problem).
...................
Shannon replied to Brent (snipped):

As far as round rock, it depends on what you mean, if they are really
round or close to it, I probably wouldn't use them, but if you just mean
irregularly shaped rocks that are not flat on two opposing sides, there is
generally nothing wrong with using these, though it will require more
mortar, and if you are in a seismic zone, depending on your design, it may
be desireable to use cement and/or rebar with the rock to give it
sufficient strength.