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The Cambrian age of the Purple Sandstone, which contains trilobites, was generally undisputed. But there were various opinions about the age of the Salt Range Formation, usually found beneath the Purple Sandstone. W. Christie, chemist for the Geological Survey of India, held that the Salt Range Formation was not of igneous origin, as proposed by C.S. Middlemiss, of the Geological Society of India. He found it to be a normal sedimentary deposit, produced by evaporation of seawater. Murray Stuart agreed with Christie that the Salt Range Formation was a normal sedimentary deposit. According to Stuart, the salt deposits in the salt range and Kohat regions were both of early Cambrian or Precambrian age. He proposed that originally both the Kohat and Salt Range Formation salt deposits had been covered by Paleozoic and Mesozoic layers. At Kohat, an overthrust had stripped the Paleozoic and Mesozoic layers away, and then the Eocene limestones were deposited atop the Cambrian or Precambrian Kohat salt. But in the Salt Range Mountains the Cambrian or Precambrian salt deposits remained covered with Paleozoic and Mesozoic layers.