The same causes that produced the layers of peat or sand, or limestone, or clay, which we find by examination of the earth's surface, are acting to-day. Coal is forming now; and so is limestone; and so is sandstone; so even is granite. But these layers or strata form very slowly, so that since man has kept historical records the thickness of new strata laid down could be measured in inches. Consequently we are only able to see the beginnings of the processes. After the materials were laid down by water or the shifting winds, or by the decay of other materials already in position, they underwent various changes. For example, many layers, instead of consisting of loose materials such as gravel, sand, or mud, are now hard stone. Sometimes this consolidation has been the result of pressure. As bed was piled over bed those at the bottom would be more and more compressed by the increasing weight of those laid down upon them; the water would be squeezed out; the particles would stick closer together. Mud, for example, might thus turn into clay; and clay, pressed harder and harder, might be converted into mudstone or shale. But there is another agency at work. We have all seen mortar hardening and binding bricks together; or cement hardening into concrete. Similarly sedimentary deposits are bound together by cements, of which there are many which exist naturally. For example, silica is a natural cement; and so is carbonate of lime; and so is peroxide of iron. All these will bind other particles together. But how do they arrive at the layers of particles? By the same action which lays down the particles themselves. They are rubbed off the places where they exist by the wind or by water. Perhaps they were laid down among the deposited particles of mud or sand. Perhaps they were brought to them by streams or rivers or lakes, and sank with the water into them. In a red sandstone, for example, the quartz grains of the rock may be often observed to be coated with earthy iron peroxide, which serves to bind them together into a rather hard stone. On the other hand, the process is often being reversed. The weather frequently conspires by frost and wind and rain to remove the binding cement, and thereby to allow the stone to return to its original condition of loose sediment. [Illustration: One of the Colossal Natural Bridges of Utah This is an instance in which water has hollowed out the lower strata, leaving a harder upper stratum partially intact.]