3 72 STRATIFIED OR SEDIMENTARY ROCKS. 



propositions. On these propositions is based nearly the whole of geo- 

 logical reasoning. 



I. Stratified Rocks are more or less Consolidated Sediments.— The 

 evidence of this fundamental proposition is abundant and conclusive. 

 1. Beds of mud, clay, or sand, as already stated, may often be traced 

 by insensible gradations into shales and sandstones. 2. In many places 

 the process of consolidation is now going on before our eyes. This is 

 most conspicuous in sediments deposited at the mouths of large rivers 

 whose waters contain abundance of carbonate of lime in solution, or on 

 the coasts of seas containing much carbonate of lime. Thus the sedi- 

 ments of the Rhine are now consolidating into hard stone (p. 82), and 

 on the coasts of Florida, Cuba, and on coral coasts generally, com- 

 minuted shells and corals are quickly cemented into solid rock (p. 156). 

 3. All kinds of lamination produced by the sorting power of water 

 which have been observed in sediments, have also been observed in 

 stratified rocks. 4. Stratified rocks contain the remains of animals and 

 plants, precisely as the stratified mud of our present rivers contains 

 river-shells, our present beaches sea-shells, or the mud of our swamps 

 the bones of our higher animals drifted from the high lands. 5. Im- 

 pressions of various kinds, such as ripple-marks, rain-prints, footprints, 

 etc., evidently formed when the rock was in the condition of soft mud, 

 complete the proof. It may be considered as absolutely certain that 

 stratified rocks are sediments. Arenaceous and argillaceous rocks are 

 the debris of eroded land, and are therefore called mechanical sedi- 

 ments or fragmented rocks. Limestones are either chemical deposits 

 in lakes and seas, or are the comminuted remains of organisms. They 

 are therefore either chemical or organic sediments. Conglomerates, 

 grits, and sandstones, indicate violent action ; shales and clays quiet 

 action in sheltered spots. Limestones are sometimes produced by vio- 

 lent action — e. g., coral breccia — sometimes very quiet action, as in 

 deep-sea deposits. 



We have already seen (p. 4) that rocks under atmospheric agen- 

 cies are disintegrated into soils, and these soils are carried by rivers 

 and deposited as sediments in lakes and seas. Now we see that these 

 sediments are again in the course of time consolidated into rocks, to be 

 again raised by igneous agencies into land, and again disintegrated into 

 soils, and redeposited as sediments. Thus the same material has been 

 in some cases worked over many times in an ever-recurring cycle. 

 This is another illustration of the great law of circulation, so universal 

 in Nature. 



Cause of Consolidation. — The consolidation of sediments into rocks 

 in many cases is due to some cementing principle, such as carbonate of 

 lime, silica, or oxide of iron, present in percolating waters. In such 

 cases the consolidation often takes place rapidly. In other cases it is 



