916 E. M. KINDLE CIIAKACTERISTICS OF MARINE CLASTICS 



which dail}^ reverses its direction. The variable direction of the cross- 

 bedding in marine elastics should serve to distinguish them from conti- 

 nental river-laid beds. If associated with ripple-mark they are easily 

 distinguished from wind-blown or dune deposits, which also show great 

 variability in the direction of their inclined beds. Wind ripple-mark is 

 characterized by relatively low and water-current ripple-mark by rela- 

 tively high crests (figure 7, a-h). 



SUMMAEY 



In summarizing the preceding discussion of the criteria of marine elas- 

 tics the significant facts which have been considered fall into three 

 groups : 1. The distinctly difi'erent behavior of fine sediments in marine 

 and in fresh waters. 2. The characteristic markings and texture of' fine 

 saline sediments developed during sedimentation and by desiccation. 3. 

 The distinguishing features of marine and continental ripple-mark. 

 Under the first head it has been shown that the extremely slow rate of 

 deposition which characterizes fresh water as compared with marine sedi- 

 mentation, when very fine sediments are involved, results in marine elas- 

 tics showing more sharply defined boundaries between different types of 

 sediment than continental or fresh-water elastics. The rate of deposition 

 differs so greatly in the two classes of deposits that the order of super- 

 position of adjacent coarse and fine beds may in some cases be in marine 

 beds the reverse of that developed in fresh-water beds. 



Deposition of finely divided sediments develops under conditions of 

 rapid sedimentation pit and mound structures on the upper surface of the 

 strata, which may sometimes be as serviceable in discriminating marine 

 beds as rain-print impressions are in recognizing subaerial deposits. 



Desiccated fresh -water mud may, under favorable conditions, be distin- 

 guished from desiccated saline mud by texture and by the character of the 

 mud-cracks. Vesicular texture frequently develops in drying saline mud 

 which is unknown in dried fresh-water mud. The mud-crack polygons 

 in saline mud remain flat or curved down at the margins. In fresh-water 

 mud the polygons curve upward, provided the tenacity of the mud is not 

 sufficient to counteract this tendency. 



Fossil ripple-mark should afford valuable aid in discriminating marine 

 deposits. In marine sandstones the current type of ripple-mark will 

 greatly predominate over the oscillation or wave type, while in lacustrine 

 deposits the reverse will be the case. The current ripple-mark of marin^^ 

 deposits may be distinguished from that of river deposits by the variable 

 current direction indicated by the former and the imiform current dire*^ 

 tion characteristic of the latter. 



