CHARACTERISTICS OF TYPE SECTIONS iioi 



continuous deposition. No sharp break either lithic or faunal should 

 occur between the members, but all should be transitional. The 

 character and origin of the strata composing the section must be 

 carefully considered, since all rocks are not of equal value as in- 

 dices of continuous deposition. 



Hydroclastic rocks are by far the most relialjle indices of depo- 

 sition, since none other are formed under so uniform an environ- 

 ment. Marine sediments, further, are more reliable than those of 

 fresh water lakes, since the latter are only temporary features of 

 the earth's surface and are preceded and succeeded by conditions 

 which will of necessity destroy the continuity of formation of strata. 

 Thus marine formations alone will serve for the erection of a 

 standard scale, all formations of a continental type, whether of 

 fresh water or of atmo-, anemo-, or pyroclastic origin, must be ruled 

 out of the standard scale. Hence the Old Red Sandstone of Brit- 

 ain, the non-marine Carbonic formations, the Newark, Potomac, 

 Dakota and Laramie formations of North America, are all to be 

 discarded in the making of a true geologic formation scale. Even 

 among marine strata, there are some which must be ruled out, as 

 not furnishing a reliable account of the progress of rock deposi- 

 tion. Thus sandstones and conglomerates, either as basal members 

 or intercalated between a series of clay or lime rocks, are almost 

 sure to introduce an element of uncertainty, if not error, into the 

 section, even if the gradation above and below is a perfect one. 

 As has been pointed out in an earlier chapter, shore-derived silice- 

 ous elastics of coarse grain, when not forming a basal sandstone or 

 conglomerate, can become widespread only by an oscillatory move- 

 ment of the land, which results in a temporary retreat and re- 

 advance of the sea. Such a change involves almost certainly a time 

 interval unrecorded in the section, but represented rather by an 

 unrecognizable break within the terrigenous member itself. An 

 example of such a formation is found in the St. Peter Sandstone 

 of central United States, a formation which in itself represents a 

 disconformity, constantly increasing in magnitude toward the north, 

 where it includes an unrecorded interval elsewhere represented by 

 from 2,000-3,000 feet of limestones. Shore deposits of all kinds 

 should be ruled out in the establishment of a typical section, for 

 they represent local conditions and, therefore, cannot furnish re- 

 liable evidence of the general progress of development. 



Deposits formed in an enclosed basin, whether marine or con- 

 tinental, are likewise unsatisfactory for purposes of establishing 

 a general scale. Such deposits at present included in the standard 

 scale of North American strata are : the Medina and Salina forma- 



