A SKETCH OF GEOLOGY. 9 



the second by shale, and the third by limestone. Argillaceous rocks are 

 easily identified by the peculiar earthy odor which they emit when breathed 

 upon, while the limestones may be detected by the aid of muriatic acid, which 

 causes effervescing when applied to them. Upon sandstone acids have no 

 effect whatever. 



STRATIFICATION. Stratification is the arrangement of rocks into different 

 layers or beds. It is a characteristic feature of all the sedimentary forma- 

 tions. Stratification can only be produced by sedimentation, and the latter 

 can, as far as geological strata are concerned, only take place in water charged 

 with solid or earthy matter, which is kept in suspense in the shape of mud. 

 Solid material has generally a greater density, or specific gravity, than water, 

 and, therefore, can be kept in suspense by the latter only so long as the lateral 

 motion of the water overcomes the action of gravitation. As soon as the 

 water ceases to move, the mud falls to the bottom, where it forms the sediment 

 which afterwards, under the enormous pressure of superimposed masses, 

 transforms into solid rock. If such sediment formation had gone on continu- 

 ously, and always under exactly the same conditions, throughout a whole 

 geological period, all the rocks of that formation would form a solid, un- 

 broken mass. But the sediment formation suffered frequent interruptions, 

 extending over shorter or longer periods, and was subjected to many changes 

 in its material. These circumstances caused a differentiation in the deposits. 

 Any interruption of the formation, or any change in the material, were indi- 

 cated by lines or planes of separation, while the different layers, thus produced, 

 were distinguished by color and texture. 



The solid or earthy materials with which the waters are charged, are derived 

 partly from the dry land by the influence of heat, frost and rain, and partly 

 by the never- resting waves of the oceans grinding up the cliffs and beaches of 

 the seashores. These agencies are employed to reduce the elevations of the 

 dry lands to the level of the oceans, and, if they were not counteracted by 

 other forces, would accomplish their task in less than six millions of years. 

 It is estimated that the average elevation of all the continents and islands does 

 not amount to fully . one thousand feet ; and, on the other hand, the work 

 accomplished by denudation is computed to be one foot in six thousand years, 

 extending over the whole area elevated above the ocean. Such calculations 

 are only mere approximations, based upon conditions which may change 

 considerably in the course of time, and should never be used for framing 

 deductions, without making great allowances. That the work of denudation 

 must have been different during the different ages of the past, can not be 

 doubted. There were periods when a very high temperature prevailed all 

 over the earth, from the poles to the equator, causing a heavy rainfall, and 

 lending the water a greater dissolving power, circumstances which must have. 



GKOL. SUR. 2 



