508 GEOLOGY 





 keeping with these general principles. Adjacent to the broad, 



shallow arm of the ocean which covered the larger part of the Mis- 

 sissippi basin (Fig. 358) there appear to have been no sources of 

 abundant sediments during most of the period. Along the western 

 base of Appalachia, mud, sand, and gravel, washed down from the 

 land, were being deposited. The coarser materials were left 

 nearer the land, while the finer were carried farther out. Alter- 

 nating beds of coarse and fine sediment indicate either (1) that the 

 adjoining land was higher at some times than at others, or (2) that 

 the climatic conditions or (3) the vegetal covering changed, or (4) 

 that waves and currents varied in their effectiveness. 



The sediments deposited at the same time in Newfoundland, 

 in the northeastern parts of Canada, and in the Ottawa basin, were 

 largely of limestone, indicating the absence of abundant debris 

 from land in these regions. About the isolated land masses farther 

 west, sand and mud, to become sandstone and shale later, were in 

 process of accumulation; but the sources of material appropriate 

 for such formations were not extensive, and the formations them- 

 selves are correspondingly limited. Conditions for the formation 

 of limestone prevailed widely in the epicontinental sea. Plants 

 and animals secreting calcium carbonate may have been no more 

 abundant far from land than near it, but away from shore their 

 shells, etc., were probably more abundant relative to the sediments 

 derived from the land. The occasional variations from limestone 

 to shale or sandstone in the interior of the continent show that 

 physical conditions were not altogether constant. 



But even during those intervals when the land was so low as 

 not to yield abundant sediments, preparation was making for 

 future formations of clastic rock. The formations of the land were 

 undergoing decay, even though the products of decay were not 

 removed. Under these conditions, thick mantles of residual earths 

 accumulate, representing the excess of rock decay over transport a- 

 tion. During such periods of rock decay a large. amount of dis- 

 integrated material is made ready for removal when uplift of the 

 land rejuvenates the streams. 



The development of the Ordovician system meant the destruc- 

 tion of an equivalent body of older rock. The old material which 



