STRUCTURE AND POSITION. 



173 



due to long -continued heavy pressure, and in still others to long -con- 

 tinued, though not necessarily very great, elevation of temperature in 

 presence of water. In these cases the process is very slow, and there- 

 fore it has not progressed greatly in the more recent rocks. 



II. Stratified Rocks have been gradually deposited. — The following 

 facts show that in many cases rocks have been deposited with extreme 

 slowness : 1. Shales are often found the lamination of which is beau- 

 tifully distinct and yet each lamina no thicker than cardboard. Now, 

 each lamina was separately formed by alternating conditions, such as 

 the rise and fall of tide, or the flood and fall of river. 2. Again, on the 

 interior of imbedded shells of mollusca, or on the outer surface of the 

 shells of sea-urchins deprived of their spines, 

 are often found attached other shells, as 

 shown in the following figures. Now, these 

 shells must have been dead, but not yet cov- 

 ered with deposit during the whole time the 

 attached shell was growing. As a general 

 rule, in fragmental rocks the finest materials, 

 such as clay and mud, have been deposited 



Fig. 136.— Serpula on Shell of an Echinoderm. 



Fig. 137.— Serpulse on Interior 

 of a Shell. 



very slowly, while coarse materials, such as sand, gravel, and pebbles, 

 have been deposited rapidly. Limestones, being generally formed by 

 the accumulation of the calcareous remains of successive generations 

 of organisms, living and dying on the same spot, must have accumu- 

 lated with extreme slowness. The same is true of infusorial earths. 



It is necessary, therefore, to bear in mind that all stratified rocks 

 were formed in previous epochs by the regular operation of agents 

 similar to those in operation at present, and not by irregular or cataclys- 

 mic action, as supposed by the older geologists. Thus, cceteris paribus, 

 the thickness of a rock may be taken as a rude measure of the time 

 consumed in its formation: 



III. Stratified Rocks were originally nearly horizontal. — The hori- 

 zontal position is naturally assumed by all sediments in obedience to 

 the law of gravity. When, therefore, we find strata highly inclined or 

 folded, we conclude that their position has been subsequently changed. 

 It must not be supposed, however, that the planes which separate strata 

 were originally perfectly horizontal, or that the strata themselves were 



