A SKETCH OF GEOLOGY. 11 



to many miles, and form, very often, extensive valleys and mountains. Strata 

 are very seldom found perfectly horizontal; they may be so for a short distance, 

 but, extending over a larger area, they will always show a certain amount of 

 curvature. These curves bending inward that is, with their convexity toward 

 the center of earth are forming troughs ; bending outward, they form arches. 

 A line running, in a series of strata, through the highest point of their arches, 

 is called their anticlinal axis, and the line running through the lowest \ oint of 

 their troughs is known as their synclinal axis. If no denudation had inter- 

 fered, we would always find the anticlinal axis to correspond with elevations, 

 and the synclinal with valleys ; but, since, by the influence of weather and 

 water the figuration of our earth's surface has greatly changed, we often meet 

 with anticlinal valleys and synclinal hills. The correct location of these lines 

 is often of great importance to the geologist in surveying a certain district or 

 country, which he can easily accomplish if he bears in mind that his proceed- 

 ing from older upon younger strata leads him towards the synclinal ; and vice 

 versa, if he proceeds from younger upon older formation he approaches an 

 anticlinal. 



Two other important features of the stratified rocks are their dip and strike. 

 Upon these, to a great extent, depends the peculiar topography of the earth's 

 surface. 



Dip is the amount of the deflection of strata from the horizontal or level 

 line ; it is measured by, or expressed in, degrees. If a layer has a dip of forty- 

 five degrees, it is bent downward, and forms with the horizontal surface an 

 angle of forty-five degrees. Wherever the dip increases to an angle of ninety 

 degrees, the strata stand on their edges in a vertical position. Strike is the 

 horizontal line drawn at right-angles to the direction of the dip. Rocks with 

 a southern or northern dip, will have an eastern or western strike. As long 

 as the dip of certain strata runs in the same direction, their strike is indicated 

 by a straight line ; but as soon as the dip changes its direction, the strike will 

 assume the shape of either a broken or curved line. Eocks with great dip 

 produce a broken undulating country, and in accordance with the curved or 

 straight lines of the strike, we find the hills and valleys respectively winding 

 or rectilineal. 



PALEONTOLOGY. 



Palaeontology treats of the animals and plants which inhabited our planet 

 during former periods, and may, therefore, be properly styled the natural 

 history of by -gone ages. From zoology and botany, it differs only in so far 

 as its objects belong mostly to an extinct fauna and flora, the remains or 

 traces of which are imbedded in the rocks and soils of the earth's crust. Palae- 

 ontology forms a science of itself ; but on account of its intimate connection 

 with geology, it is generally considered as only a branch of the latter. The 



