526 STUDIES FOR STUDENTS 



atile deposits in the geological record and much less to littoral than 

 has been customary in the past. 



In that article the distinctive features of the marine, littoral, 

 and continental deposits were incidentally mentioned, as well as 

 the features which they held in common, but the purpose was not 

 to give criteria for their distinction so much as to discuss the rela- 

 tive areal and volumetric importance which they should assume in 

 ancient sedimentary formations deposited under various geographic 

 and climatic conditions. It was urged, furthermore, that formations 

 belonging to these three zones should be sharply discriminated and 

 separated, not only because of the strikingly different conditions 

 under which they were accumulated, but also because of the funda- 

 mental importance of such distinction in many problems of paleo- 

 geographic and paleobiologic geology. 



The present article is supplementary, and it is proposed to take 

 up the subject of mud-cracks as a distinguishing feature" and to note 

 to what extent and in what associations they should be expected 

 to occur in various kinds of deposits. Such a discussion seems the 

 more pertinent since in the absence of fossils there are many detrital 

 formations in regard to which there is at present no unanimity of 

 opinion as to whether they were formed by means of either one, 

 two, or three of the following agencies, viz., aeolian, fluviatile, lacus- 

 trine, estuarine, or those pertaining to the open shallow sea. It is 

 possible that there are still other formations which have been unhes- 

 itatingly ascribed to an origin in shallow seas which may have origi- 

 nated upon the continental surfaces, since on account of the domi- 

 nance of marine deposition there has always existed a tendency in 

 the absence of fossils of land-dwelling organic forms to ascribe to 

 sedimentary formations an origin beneath the surface of the sea. 



METHODS OF ORIGIN OF MUD-CRACKS 



The closeness of resemblance between the mud- cracks which are 

 of such frequent occurrence in ancient sedimentary formations of 

 an argillaceous nature and the mud-cracks formed in modern drying 

 mud flats is such that no other origin for these ancient structures 

 has ever been, or seems likely to be, suggested, than that of exposure 

 to the air. They may be formed in any fine-grained argillaceous 



