RHYTHMS AND GEOLOGIC TIME. 347 



narrow covers of physical limitation; and there the matter rests for the 

 present. 



The rocks which were formed as sediments show many traces of 

 rhythm. Some are composed of layers, thin as paper, which alternate in 

 color, so that when broken across they exhibit delicate banding. In 

 the time of their making there was a periodic change in the character 

 of the mud that settled from the water. Others are banded on a larger 

 scale; and there are also bandings of texture where the color is uniform. 

 Many formations are divided into separate strata, as though the process 

 of accretion had been periodically interrupted. Series of hard strata 

 are often separated by films or thin layers of softer material. Strata 

 of two kinds are sometimes seen to alternate through many repetitions. 

 Borings in the delta of the Mississippi show soils and remains of trees 

 at many levels, alternating with river silts. The rock series in which 

 coal occurs are monotonous repetitions of shale and sandstone. Belgian 

 geologists have been so impressed by the recurrence of short sequences 

 of strata that they have based an elaborate system of rock notation 

 upon it. 



Passing to still greater units, the large aggregates of strata sometimes 

 called systems show in many cases a regular sequence, which Newberry 

 called a "circle of deposition." When complete, it comprises a sand- 

 stone or conglomerate, at base, then shale, limestone, shale and sand- 

 stone. This sequence is explained as the result of the gradual encroach- 

 ment, or transgression, as it is called, of the sea over the land and its 

 subsequent recession. 



In certain bogs of Scandinavia deep accumulations of peat are tra- 

 versed horizontally by layers including tree stumps in such way as to 

 indicate that the ground has been alternately covered by forest and 

 boggy moss. The broad glaciers of the Ice age grew alternately smaller 

 and larger — or else were repeatedly dissipated and reformed — and their 

 final waning was characterized by a series of halts or partial readvances, 

 recorded in concentric belts of ice-brought drift. Of these belts, called 

 moraines of recession, Taylor enumerates seventeen in a single system. 



In explanation of these and other repetitive series incorporated in 

 the structure of the earth's crust, a variety of rhythmic causes have been 

 adduced; and mention will be made of the more important, beginning 

 with those which have the character of original rhythms. 



A river flowing through its delta clogs its channel with sediment, 

 and from time to time shifts its course to a new line, reaching the sea 

 by a new mouth. Such changes interrupt and vary sedimentation in 

 neighboring parts of the sea. Storms of rain make floods, and each flood 

 may cause a separate stratum of sediment. Storms of wind give destruct- 

 ive force to the waves that beat the shore, and each storm may cause 

 the deposit of an individual layer of sediment. Varying winds may 



