THE ORDOVICIAN PERIOD 341 



This implies clear water, for, although shell-bearing ani- 

 mals are often abundant in turbid waters, their remains are 

 there mixed v/ith so much mud or sand, that shale or sand- 

 stone is the resulting rock. Thus along the western flank 

 of Appalachia there is less limestone in the Ordovician system 

 because the land supplied greater quantities of sand and clay. 



Where lands are high they are more rapidly eroded, and 

 when the mountains are near the sea a correspondingly rapid 

 accumulation of coarse sediments is likely to take place off 

 shore. When, however, broad low plains clad with vegeta- 

 tion border the seas, it may happen that little material is worn 

 from the surface thus protected, and likewise little sediment 

 may be washed into the sea in that vicinity. Such considera- 

 tions as these serve to explain the fact that the period is 

 represented by over 4000 feet of strata in eastern Ten- 

 nessee, but by only a few hundreds of feet in Missouri. 

 Similarly, there may be differences in the rate at which cal- 

 careous sediments accumulate, for in warm, shallow waters 

 shell-bearing animals are likely to be far more numerous than 

 in cold waters and far from shore. 



Subsequent changes in the sediments. The Ordovician 

 sediments were laid down in nearly horizontal beds, and were 

 almost entirely buried by sediments deposited at a later time. 

 Since then they have been consolidated into hard sandstone, 

 limestone, and shale. In some places they have been folded 

 or bulged up in such a way that they have been uncovered 

 by the erosion of the land. Thus the outcrops of Ordovician 

 rocks are now found adjacent to those of Cambrian age. In 

 the Appalachian Mountains these outcrops lie in parallel 

 bands, while on the other hand they form rings about certain 

 upraised masses of older rocks in the northern and western 

 states, as in the Adirondacks, in Missouri, and in the Rocky 

 Mountains. On the Pacific coast, as well as in New England, 

 the Ordovician rocks have been severely metamorphosed, so 

 that it is now a matter of extreme difficulty to distinguish them 

 at all, 



