CALCIUM CARBON DIOXIDE, FOR EACH CARBON DIOXIDE MOLECULE BICARBONATE CALCIUM CARBONATE ESCAPING INTO CAVE AIR, AN REACHING CAVE EQUIVALENT MOLECULE OF CALCIUM AIR CARBONATE MUST BE REDEPOSITED AS SOLID DRIPSTONE, FLOWSTONE, ETC. It was thus that phreatic water, charged with soil acids, percolated century after century through cracks in the marble. The acids ate away at all exposed rock surfaces—sideward, downward, and upward. (The solution rills in the original Ghost Room ceiling reveal the upward dissolving of water-filled cavities, see illustration page 9). To fully understand this, we must recall that the marble is 93 percent _calcium carbonate_ (CaCO₃). To dissolve it, carbonic acid mixes with the calcium carbonate to form an unstable liquid compound called _calcium bicarbonate_, (CaH₂(CO₃)₂). The removal of solid calcium carbonate in a liquid is the key cave forming process and is called _solution_ (see illustration page 10). In Watson’s Grotto we find several examples of early crack-widening by phreatic solution (see illustration page 9). The enlarged cracks allowed faster movement of water against an increased surface area, and a subsequent increase in solution activity. Partitions between them fell apart and were dissolved. A series of water-filled passages evolved deep underground. Their pattern and orientation followed the pre-cave network of joints and cracks in the original strata. Gradually the openings were further enlarged into the cave system we know today. There is more to it than that, of course. You may ask, “Why aren’t there caves continuously throughout the belt of marble? The joints and cracks are everywhere. And certainly all the marble near the surface has been subjected to ground water action at some time or another. Why are Oregon Caves limited to one particular part of the marble belt?” The answer to this involves several considerations. To begin with, we do find small cavities and solution cracks throughout the exposed marble. So there has been varying degrees of solution activity nearly everywhere, although not sufficient to produce caverns comparable to Oregon Caves. Secondly, we must reconsider the mineralized water, calcium bicarbonate. We called it an _unstable_ compound, meaning it will alter readily with slight changes in conditions. Once the amount of carbon dioxide dissolved in the water has united with an equivalent amount of calcium carbonate (marble), the solution is saturated. No more marble can be dissolved until additional carbon dioxide is absorbed by the water. If the solution loses some of its carbon dioxide into the air, then an equivalent amount of calcium carbonate must be redeposited as solid stone. The balance can be delicate. In an underground pool of mineralized water, _solution_ may be going on at one end of the pool and _deposition_ at the other.