“So goes the legend. I suppose you have heard it?”
“Yes! and it rather confirms my theory. Legends have always a base in fact; and whatever cause gave rise to the myth of St. Patrick and the King of the Snakes, the fact remains that the legend is correct in at least one particular—that at some distant time there was a lake or pond on the spot.”
“Are you certain?”
“A very cursory glance satisfied me of that. I could not go into the matter thoroughly, for that old wolf of mine was so manifestly impatient that I should get to his wild-goose chase for the lost treasure-chest, that the time and opportunity were wanting. However, I saw quite enough to convince me.”
“Well, how do you account for the change? What is your theory regarding the existence of limestone?”
“Simply this, that a lake or reservoir on the top of a mountain means the existence of a spring or springs. Now springs in granite or hard slate do not wear away the substance of the rock in the same way as they do when they come through limestone. And moreover, the natures of the two rocks are quite different. There are fissures and cavities in the limestone which are wanting, or which are at any rate not so common or perpetually recurrent in the other rock. Now if it should be, as I surmise, that the reservoir was ever fed by a spring passing through a streak or bed of limestone, we shall probably find that in the progress of time the rock became worn and that the spring found a way in some other direction—either some natural passage through a gap or fissure already formed, or by a channel made for itself.”
“And then?”
“And then the process is easily understandable. The spring naturally sent its waters where there was the least resistance, and they found their way out on some level lower than the top of the hill. You perhaps noticed the peculiar formation of the hill, specially on its west side—great sloping tables of rock suddenly ended by a wall of a different stratum—a sort of serrated edge all the way down the inclined plane; you could not miss seeing it, for it cuts the view like the teeth of a saw! Now if the water, instead of rising to the top and then trickling down the old channel, which is still noticeable, had once found a vent on one of those shelving planes it would gradually fill up the whole cavity formed by the two planes, unless in the meantime it found some natural escape. As we know, the mountain is covered in a number of places with a growth or formation of bog, and this water, once accumulating under the bog, would not only saturate it, but would raise it—being of less specific gravity than itself—till it actually floated. Given such a state of things as this, it would only require sufficient time for the bog to become soft and less cohesive than when it was more dry and compact, and you have a dangerous bog, something like the Carpet of Death that we spoke of this morning.”
“So far I can quite understand.” said I. “But if this be so, how can the bog shift as this one has undoubtedly done? It seems, so far, to be hedged with walls of rock. Surely these cannot move.”
Sutherland smiled. “I see you do apprehend! Now we are at the second stage. Did you notice as we went across the hill side that there were distinct beds or banks of clay?”