RHYTHMS IN SEDIMENTATION 805 



and are interbanded with fine-grained dark gray limestone. Detailed 

 examination brings to light the fact that the cryptozoa were periodically 

 blanketed by a soft lime mud, following which new colonies would start 

 to grow on the layers of mud. The colonies were favored by clear water 

 and became cemented crusts. These crusts, while yet thin, were liable 

 to become broken up by wave action and form intraformational con- 

 glomerates. The thicker and stronger colonies had their existence ter- 

 minated, not by wave action, but by becoming smothered beneath the 

 next heavy invasion of mud. In some localities, however, the life of the 

 colonies was probably terminated by draining away of water and drying, 

 as shoAvn by mud-cracking. Stronger wave action and influx of mud 

 were thus the features determining the dark gray limestones; gentler 

 wave action and clear water permitted the growth of cryptozoa. These 

 conditions alternated in a markedly rhythmic fashion. The nature of 

 the composite rhythm is shown well in the lower portion of the section. 

 Many fine laminations, hardly visible in the photograph, make up a 

 minor rhythm. Four or five repetitions of the minor rhythm led up 

 to a icrescendo favoring progressively longer times for the growth of 

 cryptozoa. Then a sudden change took place to that phase favoring the 

 accumulation of lime mud. In the upper third of the section the rhythm 

 is seen to change in character ; a lesser thickness is deposited in each 

 cycle, the cryptozoan layers are sharper and thinner, the ^crescendo seems 

 to have disappeared. 



To what degree does this series of beds represent continuous or dis- 

 continuous deposition? Each cryptozoan colony doubtless lived a con- 

 tinuous life until it was smothered. The mud appears to have come in 

 suddenly, since all parts of the colony stopped growing at once, both the 

 base and top of the hemispherical turbans. There may thus have been 

 a continuous deposition of alternately organic and inorganic nature, but 

 at a variable rate. On the other hand, alternate fill and scour may have 

 taken place during the deposition of the mud-beds. The rather abrupt 

 change in the character of the rhythm may signify a loss of record 

 during a time when the cryptozoan growth was stopped and before the 

 colony was buried. Such a period of lost record would account for the 

 sudden change in character, but on the whole the evidence suggests for 

 this section the other explanation of sudden death by burial. The regu- 

 larity and composite nature of the rhythm favors interpretation as a 

 record of climatic oscillations and the whole section may represent a 

 phase of a larger rhythm marked by a slowly subsiding bottom. T. W. 

 Vaughan has shown that such a condition of submergence favors the 



