We still have to consider a third mode of occurrence of waters within the earth. Many formations, like granite and other types of crystalline rocks are neither in definite layers, nor are they sufficiently porous to allow water to really flow through them. Where such rocks extend far down from or near the surface, how does rain water descend? It does so along cracks or fractures (both joints and faults) which we have learned are almost universally abundantly present in all hard rocks in the upper (or zone of fracture) portion of the earth's crust. Joint cracks are generally very irregular in direction and spacing, while fault fractures are usually fairly regular and straight. Many cracks are not wide enough to allow anything like good passageways for water, while others are sufficiently open to allow water to travel along them for hundreds, or even thousands of feet. In canyons of the West, springs not rarely emerge from the bottoms of great, nearly vertical ledges of granite and other hard crystalline rocks, the waters certainly having entered the rocks hundreds, or even some thousands of feet, higher. In rocks of the kind here considered it is evident, then, that the movements of subterranean waters must be mostly exceedingly irregular and usually not in great quantities. In many deep mines of the world, underground water causes little or no trouble except often near the surface. Occasionally a shaft or tunnel strikes a prominent joint or fault fracture filled with water. What we might really call underground streams may occur only under exceptional conditions in rocks other than limestone, but in limestone they are not uncommon because the slow solubility of the rock allows underground waters to slowly enlarge the passageways to form distinct channels. Echo River, which flows through Mammoth Cave, is a fine case in point. Most water by far which emerges as springs, was at one time surface water. A simple, but common case is where rain water soaking through porous soil (e.g., sand) or rock, sinks to the top of an underlying impervious layer (e.g., clay) along whose surface it flows until it reaches the side of a valley where a spring results. In fact, wherever the water table is crossed by the surface of the ground, water must either seep or flow out. Where underground streams which are common in limestone regions reach the surface on hill or valley sides, springs result. Another source of springs is where under proper conditions of slope a porous rock layer, charged with water well below the surface, appears at a lower level than its source of water. Still another type of spring is where a fissure or fracture crosses a water-bearing layer in which the pressure is great enough to cause the water to rise to the surface along the relatively open fissure or fracture.