MATERIALS OF THE EARTH 39 



Organically formed ( Calcareous rocks, most limestones. 



Non-clastic ] Siliceous rocks, siliceous oozes, sinter, etc. 



\ Carbonaceous, coal, etc. 



Original structure of sedimentary rocks. As originally de- 

 posited, the gravel, sand, mud, etc., are in beds. The beds may 

 be of different material, or all may be essentially alike. In the 



Fig. 16. Open anticlinal fold, near Hancock, Md. (U. S. Geol. Surv.) 



latter case, there are often thin partings of some different material. 

 Thus layers of sandstone may be separated by films of mud, layers 

 of shale by a little sand, etc. These partings may record stages 

 when the water was rougher or quieter than usual, or when it had 

 an excess or a paucity of sediment. 



At the time of their deposition, the beds of sediment conformed 

 in a general way to the slope of the bottom where they were de- 

 posited. Since the slope of the sea bottom near shore is very 

 gentle as a rule, the beds of sediment are, in most cases, nearly 

 horizontal at the time of their deposition. Their angle of slope 

 is rarely so much as 20, and is commonly less than 5. 



Secondary structure of sedimentary rocks. Many sedimentary 

 rocks have lost their original position through crustal movements, 



