34 NATURE STUDY REVIEW [9 : 2— Feb, 1913 



calcium carbonate in solution becomes limestone. When the sea 

 water is evaporated in a basin the salt and gypsum which it con- 

 tains are precipitated and form beds of these rocks. 



The loose plastic materials become hard and firm by being 

 cemented by various materials and squeezed together by pres- 

 sure. The coarse material such as the pebbles of a conglom- 

 erate and the coarse sandstones must be cemented to become 

 hardened, while pressure will compress the soft muds and cal- 

 careous material into firm rocks. Some of the more common 

 cements are calcium carbonate, iron oxide, silica, and sometimes 

 mud. These cements may be' introduced into the rock while it 

 is yet in the sea or they may be buried and consolidated while 

 under the water and after they are raised above the sea bottom 

 to form land the water may deposit the cement in the openings 

 between the particles. As more material is brought into the sea 

 and deposited, it buries that below, and the weight of the overly- 

 ing beds is in most instances sufficient to consoHdate the beds of 

 soft material into firm rocks. The enormous pressures that the 

 rocks are subjected to when they are folded into mountains 

 also aids in consolidating the rocks, in fact, the pressures and 

 heat developed during the formation of mountains are sufficient 

 to change the rocks so much that they become metamorphosed 

 into very different rocks which will be described later. It is thus 

 that the soft sands and muds become the usually firm hard rocks 

 that we know. 



The sediments are always in beds or layers and are com- 

 monly spoken of as being stratified. (See figure 2.) It is in 

 this feature that they differ from the igneous rocks. Their com- 

 position is also very different from that of the igneous rocks. 

 The limestones and dolomites are carbonate rocks which are 

 easily dissolved in acids and are also usually soft, about three or 

 four in the scale of hardness. The shales and sandstones also 

 differ in composition from the igneous rocks and can also be 

 readily distinguished from the igneous rocks and each other, the 

 former by its well marked bedding planes, its softness and clayey 

 odor, and the latter by its being composed of rounded grains 

 (these may be of quartz, feldspar and various other minerals), 

 its friableness and its bedded character. 



There are all gradations between the various sediments so 

 that one could get a complete series between any two sediments. 

 The shales may be calcareous and with an increasing amount of 

 calcium carbonate and a decreasing amount of clay they would 

 pass into a limestone, or if sand became abundant it would be 



