"The relative hardness of the different kinds of rock," was the reply. "Not to seem too technical, the top stratum, that is to say the rock immediately under the soil of this plateau, while quite hard, is very thin, and underneath it are various other layers of rock, some fairly hard and others very soft. The Colorado River has a very swift current, and once it had cut through the hard rock on the top it quickly ate its way downward through the soft limestones and sandstones below. But some strata were quite hard and these, resisting the water, formed the terraces which you see on every hand." "But I still don't understand," said the boy, "what it is that gives them such curious shapes. I can see how a hard rock would make a terrace, but why aren't the lines all regular?" "Just because it has been done by water. Sandstone, you know, is made of sand, pressed, and sand is all sorts of rocks ground down fine. So every handful of sand may contain particles of a dozen different kinds of rock, and if there was any difference in the hardness of the rock of which the sandstone was made, or any difference in the pressure while it was being made, each difference would show up by its greater or less resistance to the action of wind and water. So, you see this bit is hard and cuts slowly, that bit soft, and cuts rapidly, giving a carved effect." "But if it all follows a regular rule, why does it look so unnatural?" "That is easy," replied his informant. "The strata are regular--that is what makes the masses look like buildings done by hand, there is a sense of proportion, but they look unnatural because the ground plan is capricious, the water having found its way to the bottom of its thousand canyons by the irregular and complicated way of least resistance." "And the colors seem so glaring and so strange!" "I will explain those to you after dinner," said the topographer, "and, by the way, it is nearly time we returned to the hotel or we shall be late. I can show you how the various reds are due to iron in the rock--you remember how a rusty nail stains everything red?--and other iron compounds give the green, while the blues of the slates and the dark belts of hornblende all play their part." Masseth was as good as his word and all through the time spent in the dining-room he interested the boy in the country by his vivid descriptions of how all these rocks had first been made, then reduced to sand and built up again, and how the Colorado River was fast tearing them down and carrying them away to be built up somewhere else in some other way. "Then geology isn't all over!" exclaimed Roger in surprise. "I always thought of it just as a sort of history of things that happened a great while ago." "Geology is happening right along," said Masseth, "and that's why it is so necessary to do this work and find out both what has been and what is going to be, even though it is both difficult and arduous." [Illustration: AN AWKWARD COUNTRY TO WORK IN.