12	PRECIOUS STONES.
	all-important factor, time. But in Nature's laboratory it is quite different, and the rock-forming minerals of the lava are slowly decomposed, a small portion of the material being carried deeper into the rock. Now, a lava frequently contains steam holes, or vapour cavities, and if the solution meet* one such in its downward path, it is carried into the cavity by osmosis. Usually the first solution to enter is one derived from the attack of the water on one of the ferro-magnesian minerals, and once it is in the cavity a thin film of a mineral (in most cases one allied to Serpentine) is deposited on the wall of the cavity, and forms what is called the " priming." Dr. Heddle showed this priming to consist in most cases of Celadonite. The thickness of this layer may vary from a mere film to one entirely filling the cavity. If the Celadonite continued to be deposited along with silica in the form of Chalcedony, it often took the form of moss­like growths, surrounded by the clear Chalcedony, giving rise thus to a Moss Agate. If the Celadonite and Chalcedony were intimately mixed Prase or Heliotrope would be formed. Later silica in solution was carried into the cavity by osmosis, and deposited next inside the priming, or only separated from the Celadonite by a thin layer, to be described immediately. The deposition of the silica took the form of Chalcedony, and was very probably brought about by the escape of the solvent. Often the decomposition of the ferro-magnesian silicates was closely followed by the disintegra­tion of the felspars of the rock, and the resulting products carried into the cavity and deposited immediately within the Celadonite, as a layer of one of the Zeolites. Rarely the entire cavity might be filled with Zeolite. All three of these minerals form coatings of practically uniform thickness.