Brisbane Tuff... or Porphyry?

Quoted text begins.I am completely gone over porphyry here in Missouri. It
occurs here in boulders exactly in this color of royal
purple.
http://en.wikipedia.org/wiki/Porphyry_(geology)
End of quote.

Hi Bill,

Thanks for the links to the photos - Egyptian porphyry is certainly a
beautiful stone, but it's not Brisbane Tuff: http://tinyurl.com/a7uox

Brisbane Tuff has been quaried at several sites in Brisbane, and the ready
source of rock was probably one of the reasons (in addition to a supply of
fresh water, and a pleasant site in the river's tidal reaches) that the
site of the Moreton Bay penal settlement (later called Brisbane)was
chosen in 1825.

The early stone buildings were constructed from Brisbane Tuff building
stone, mortared with lime sourced from Aboriginal shell middens.
Many people use the term "Porphyry" to describe this stone.
True porphyry is an igneous rock, but the Brisbane Tuff is really a
welded ignimbrite (fire-rain-rock), a volcanic sediment.

Brisbane Tuff was deposited apparently in a single cataclysmic event
200 million years ago, the result of a pyroclastic flow that filled
the river valleys with a 400ft thick layer of volcanic ash.
Brisbane River found its way back through the rock, forming the Kangaroo Point
Cliffs famed by rock climbers: http://tinyurl.com/c3897

(This Wiki states that it is rhyolitic lava - well, that is only
partly true. It was certainly siliceous magma, but didn't get to
form lava because it was ejaculated so violently as a "glowing cloud"
eruption before depositing and compacting under its own weight,
then becoming partly cooked using its latent heat.
I have read that the contact between the original ground
(and river bed) surface is marked by weak sediment and charred logs.)

As far as I know, there are two uses of the term "porphyry":
* purple (dye got from sea slugs to colour the royal Greeks' togas)
* the rock described as porphyry because of its porphyritic texture
(large crystals in a fine-grained ground mass)

Well, Brisbane Tuff can have a lovely mauve hue (its colours range
in pastels purple, green, pink & white), and there are many
particles of flattened pumice that resemble crystals - but it's not porphyry.

Still, the name "porphyry" or even worse "palfrey" has stuck, and I
get strange looks when I use the correct term "Brisbane Tuff".

Its hardness can vary, but it is mostly like hard sandstone, and it
does not take a polish. It splits easily along sedimentary planes,
but has an additional 135 deg. cleavage structure caused by heating
after deposition as I described previously.

Some of Brisbane's buildings http://tinyurl.com/8qty7 were made from
Benedict Stone http://tinyurl.com/9x8mx an American
cast-stone product http://tinyurl.com/9dvxz made from Brisbane Tuff
aggregate bound with coloured cement matrix.

Simon