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Lasallia (or Umbilicaria) pustulata. The 'pustules' or lumps in the centre of the rosette are characteristic of the species, sometimes called Toadskin Lichen because of this. It is edible when carefully prepared, (mainly in Asian cuisine) and also been long known as a survival food in the Arctic to ward off starvation. However it is not very likely to become the latest 'must have' delicacy judging by what Ernest Thomson Seton wrote of it:

Rock Tripe... found on the bleakest, highest rocks in the northern parts of this continent [North America]. There is a wonderful display of it on the cliffs about Mohonk Lake, in the Catskills. Richardson and Franklin, the great northern explorers, lived on it for months. It must be very carefully cooked or it produces cramps. First gather and wash it as clear as possible of sand and grit, washing it again and again, snipping off the gritty parts of the roots where it held onto the mother rock. Then roast it slowly in a pan till dry and crisp. Next boil it for one hour and serve it either hot or cold. It looks like thick gumbo soup with short, thick pieces of black and green leaves in it. It tastes a little like tapioca with a slight flavoring of licorice. On some it acts as a purge.
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It has also been used to produce a red dye.