666 PRINCIPLES OF STRATIGRAPHY 



tidal flats and marshes has already been sketched. (See Chapter 

 XI.) It need only be noted here that the deposits accumulating 

 in the lagoon are the finer sands and muds brought by the tidal 

 currents, and the sands blown across from the beach. Though 

 stratification may be well marked in these lagoon deposits, the evi- 

 dence of agitation by waves or currents is slight. Cross-bedding 

 structure is not characteristic of such deposits, but ripple marks, 

 of the oscillation type, i. e., symmetrical, sharp crests with broad, 

 rounded troughs, may be formed. A characteristic feature of these 

 deposits is the presence of the eel grass leaves which penetrate the 

 strata in a vertical direction, having been instrumental in the pre- 

 cipitation around them of the suspended material, through a check- 

 ing of the velocity of the current. In the last-formed deposits of 

 the lagoons organic matter will predominate and peat beds — the 

 coal of the future — mark the transition through the shore zone to 

 terrestrial conditions. Of older deposits of this type the Low- 

 ville limestone has already been cited. This is a formation of fine 

 lime mud deposited around vertical stems or branches of a marine 

 organism, probably not a plant, however. Thus the resulting calci- 

 lutyte is penetrated by numerous vertical tubules, now largely oc- 

 cupied by crystalline calcite, and producing on the bedding planes 

 the so-called bird's-eye structure which gave the formation its orig- 

 inal name. 



3. Epicontincntdl Seas and Mediterraneans. Those portions of 

 the littoral district which extend into the land as arms or partly 

 enclosed embayments of no great depth, without being estuaries of 

 great rivers, are of considerable importance to the stratigrapher, 

 since epicontinental seas of this type were very characteristic of 

 many geologic epochs. Their chief significance is, however, a faunal 

 one, since sedimentation, except in special cases, does not differ 

 much from that upon the continental shelf. Nearly landlocked epi- 

 continental seas within pluvial districts, such as Hudson Bay or the 

 Baltic Sea, do not differ appreciably from the littoral belt of the 

 open ocean, but where such enclosed seas occur in arid climates the 

 greater density and increased salinity of their waters will influence 

 perceptibly not only the fauna and flora, but also the deposits. If 

 the intracontinental sea of the arid or semiarid climate is a med- 

 iterranean, i. e., if its center extends to depths much greater than 

 its outlet, the peculiarities of the deposits will be emphasized. The 

 Black Sea forms an instructive example of the conditions found in 

 an almost completely enclosed water body of great depth in a semi- 

 arid region. According to Andrussow (5; 21:200), the superficial 



