Iain Stuart wrote:
>
> Sounds like a Chilean Mill which were quite common in Victoria although I
> cannot think of any which have been excavated or recorded in detail

The two devices are very similar.  In both there is a flat circular
pavement with a vertical axle in the center and some sort of curb around
the outside edge.  The difference is that the in the arrastra heavy
stones (suspended from a horizontal bar attached to the central pivot)
are dragged around the pavement.  The Chili Mill is a direct descendent
of the arrastra.  The difference is that wheels roll around the central
pivot to grind ore on top of the lower surface.  Most arrastras in this
area have stone patios and curbs while the Chili Mills (one is still
installed in a mill just down from my house) are generally made entirely
of iron.

If you only have circular stone patios and curbs, the difference can be
seen on the wear patterns of the patio stones.  Drag stones make nice
clear arcs (good enough to determine patio dimensions of disassembled
arrastras) while a roller on stone is more likely to create more of a
random pattern of wear marks from downward crushing rather than
dragging.

It is critical not to focus too much on the arrastra itself.  It is only
the fine-grinding appliance in a milling flow that has been widely
varied.  Also, they have been linked to about every imaginable power
source including water, animals, and internal combustion engines.

A final note- be careful working around these things since they were
often used in conjunction with amalgamation and associated sediments may
contain high levels of mercury.

Ron Reno