8 



SHORTER CONTRIBUTIONS TO GENERAL OEOLOCY, 1921. 



with the nia.ximiiin of the oscilhitioii that ])ro- 

 (liiced this change of l)ase-le\'el. Some part of 

 the ocean bottom must remain sul (merged dur- 

 ing a rise of base-level, and there the maximum 

 will be recorded in the sedimentary beds. In 

 the Seaman well a surface corresiionding to this 

 maximum may be included in the interval 

 represejited by the sample from 8,040 to 3,050 

 feet, in which a maximum of sand would (cor- 

 respond to a maximum of shallowing. For 

 this reason the ])oimdary between imits B and 

 C has been placed hi the middle of that interval. 

 This boundary is of course not a disconformity 

 and is so rejjresented onlj* for convenience. A 

 name to describe this surface would be use- 

 ful. The name "akinetic siu'face" is pro- 

 posed and defined as the surface in a sedimen- 

 tary rock which was the outer surface of the 

 lithos))here at some ])lace at the moment an 

 oscillation of base-level at that place passed 

 through its maximum. The word akinetic is 

 derived from the Greek word Kit'(o), I move, 

 and the ])rivative ]>r(>fix a. Although tlie in- 

 terpretation here given of the boundary 

 between the two formations is a possible in- 

 terpretation of the observations recorded, it is 

 not inevitable. There may be a considerable 

 time interval, not represented by sediments, 

 between units B and C, and the ap])arent con- 

 tinuity may be due merely to the reworking 

 of material at the top of imit C into the basal 

 sediments of imit B. 



A fact which causes doubt as to the true po- 

 sition of (lie boundary between units B and C 

 in the Seaman well and points toward its oc- 

 currence about the base of the sandstone shown 

 at 3,075 feet is the appearance in the sample 

 from 3,090 to 3,100 feet of shale blacker than 

 any above it and of a type which is distinctly 

 characteristic of unit C. The exact position in 

 tliis well of the boundary between units 1^ ami 

 C witiiin an interval of 30 to 40 feet therefore 

 lemains uncertain. 



Ill I he liU(hl well the boundary is well dehned 

 by a lypical l)ed of autochthonous glauconite, 

 with much sulphide, some phosphate nodules, 

 very t'oarsc iussils or fossil fragments, and a 

 httie sand. 



DIFFERENCE BETWEEN THE SHALES OF UNIT B 

 AND THOSE OF UNIT C. 



The pi-('vailing shales of unit B in the Seaman 

 well a|)pear in thin section predominantly 



finely granular or felty in appearance, passing 

 where coarser into micaceous and sandy shales. 

 They are of an even brown color and full of 

 brown fragments. These fragments usually 

 increase in coarseness with the coarseness of the 

 sediment, and some of the larger ones can be 

 recognized as fragments of plant matter. .Sul- 

 phides are common and are largely associated 

 with the plant fragments. The lime in these 

 shales, though locally very abundant, is generally 

 disseminated as a fine dust through the mass. 



The prevailing shale of unit C as seen in thin 

 section may be described as of the Marble Falls 

 type -that is to say, tlie matrix instead of being 

 even and granular has more of a flocculent ap- 

 pearance, being amorphous rather than com- 

 posed of fine mineral matter. The color is 

 usually a blacker brown and, in correspondence 

 with tlie flocculent character, is less even than 

 in unit B. Organic matter is undoubtedly 

 abundant but does not appear as definite units 

 hut rather as a vague, irregular stain, indistin- 

 guisliable from the argillaceous matter. Sul- 

 phid(>s, though present, are therefore not gen- 

 erally as clearly associated with organic ele- 

 ments as in unit B. Whether there is generally 

 more or less sulphide in unit B or imit C could 

 not be determined by the eye. especially on 

 account of tht^ great difl'erence in the amount 

 of this constituent in different fragments. The 

 determination of the part played by organic 

 matter in these shales and of its relation to the 

 amount and possibly to the mannei' of occur- 

 rence of the sulphide is one of the problems 

 which I hope to take up later. The most dis- 

 tinguishing characteristic of shale of the unit C 

 tyi)e is the occurrence in it of lime in coarse ir- 

 regular fragments and grains, many of them 

 clearly fragments of shells. With the increase 

 of the amount and coarseness of the lime and of 

 the fossil fragments or fossils the shale passes 

 by imperceptible gradations into black lime- 

 stone " and then into gray limestone. With 

 these ilifl'erences in microscopic character be- 

 tween the shales of units B and C in the Sea- 

 man well goes a difference in their color to the 

 unaided eye. Those of unit B arc generally not 

 quite as black as those of unit C, being more of 

 a i)lue-l)lack. They also usually efiervesce less 

 activelv in acid. 



"See Udden, .1. \., and Wailc, V. V., MitTo-scopic chararl eristics of 

 the Bend and ttie Elleuljurger limestones, Texas Bur Econ. (leology, 

 191 a. 



