RHYTHMS IN SEDIMFJNTATION bUo 



Turning to marine miiclstones, it \\()iild seem that here are the most 

 favorable conditions for a continuous record. They seem well adapted, 

 especially where marked]}^ alternating witli heds of lime or silt, to give 

 a basis for the aJialysis of rhythms. 



An instructive illnstration of what is probal)iy in some beds a con- 

 tinuous record and in other beds a record not greatly discontinnons is 

 found in the Martinsburg or Hudson Eiver slates. The writer has studied 

 them especially in eastern Pennsylvania, where the commercial value of 

 certain beds has resulted in extensive quarrying and numerous exposures. 

 The formation is here in considerable part a silt rock. This facies. 

 where studied in the neighborhood of the Lehigh Eiver^ is divided into 

 beds from several inches to several feet in thickness, constituting a 

 ribboned slate. An illustration showing this feature is given in plate 43, 

 figure 1. In a single exposure the rhythm in bedding tends to great 

 regularity. Through most of the formation the ribbons are reduced to 

 mere parting planes between beds of silty character. In places thicker 

 l^eds may be almost uniform in texture, but have thin bands of lighter 

 color running through them w^liich appear to be due to a slightly lower 

 content of carbonaceous mud. Such faint color bands may not destroy 

 the value of the slate. In some localities these bands of slightly lighter 

 color alternate with a very regular spacing with bands which vary in the 

 other direction, containing softer and blacker material. All these varie- 

 ties of divisions in bedding are thus seen to be due to related causes of 

 regular recurrence. ' Turning to the notably ribboned slates, we may find 

 in them the key to the significance of the bedding. The ribbons often 

 consist of a band of soft l^lack mud-rock overlain by a band of nearly 

 clean sand. The mixture of the two would appear to give the normal 

 composition shown in the intervening beds. This sequence is illustrated 

 in plate 43, figure 2. 



The layer of sand is often subdivided into laminae and occasionally 

 shows ripple-mark. Kindle has shown by experiment that in a silt de- 

 posited after stirring up in fresh water the coarser particles are deposited 

 first, whereas in salt water the coagulation into nuclei is such that the 

 slimes are deposited first, and the very fine sand follows.^- These rib- 

 boned slates indicate, consequently, tliat at recurring intervals the bot- 

 tom of a shallow Ordo^ician sea \\-as stirred up by waves of unusual 

 intensity. On the dying down of the wave action the sediment which 

 was held in the water at that place settled, making the ribbon of this 

 character, ft can not be assumed that the recurrent strong wave action 



32 E. M. Kindle: Diagnostic characteristics of marine elastics. Paper presented to ttie 

 Geological Society of America December 29, 1916. 



