STRATIFICATION. 



91 



have been made by stones, gravel, or sand, transported by the winds, 

 water, or ice, or by any other cause of movement. Figure 93 A repre- 

 sents scratches made by glacial action. 



3. Positions of Strata. 



The natural positions of strata as formed, and the positions result- 

 ing from the disturbance or dislocations of strata, are two distinct 

 topics for consideration in this place. 



1. The natural positions of strata as formed. — Strata in their 

 natural positions are commonly horizontal, or very nearly so. The 

 level plains of alluvium and the extensive delta and estuary flats show 

 the tendency in water to make its depositions in nearly horizontal 

 planes. The deposits formed over soundings along sea-coasts are 

 other results of sea-action ; and here the beds vary but little from 

 horizontality. Off the coast of New Jersey, for eighty miles out to 

 sea, the slope of the bottom averages only 1 foot in 700, — which no 

 eye could distinguish from a perfect level. As the processes of the 

 present period along coasts illustrate the grand method of rock-accu- 

 mulation in past time, it is plain that strata, when in their natural 

 positions, are very nearly, if not quite, horizontal. Over a consider- 

 able part of New York and the States west and southwest, and in 

 many other regions of the globe, the strata are actually nearly hori- 

 zontal at the present time. In the Coal-formation, the strata of 

 which have a thickness, as has been stated, of five to fifteen thousand 

 feet, there is direct proof that the beds were horizontal when formed ; 

 for in many of the layers there are fossil trees or stumps standing in 

 the position of growth, and sometimes several of these rising from 

 the same layer. Fig. 94 represents these 

 tilted coal-beds c, c, with the stumps s, s, s. 

 Since these trees must have grown in a verti- 

 cal position, like all others, and as now they 

 are actually at right angles to the layers, and 

 parallel to one another, they prove that the 

 beds originally were horizontal. The position 

 of shell -accumulations and coral-reefs in modern 



seas shows, further, that all limestone strata must have been nearly 

 or quite horizontal when they were in the process of formation. 



Fig. 95. 



Fig. 94. 



In sedimentary deposits, however, some variation from horizontality 



