46 



SCIENCE 



[N. S. Vol. LIII. No. 1359 



" It is my opinion that the fauna of the 

 Cannonball marine member of the Lance 

 formation indicates that the formation be- 

 longs within the hiatus discussed by Stephen- 

 son and below the line which he postulates as 

 separating Cretaceous from Eocene. It is 

 certainly somewhat younger than the zone of 

 Exogyra costata and most probably consider- 

 ably older than the Midway formation" 

 (p. 15). 



The regularly bedded marine Pierre strata 

 pass unbroken into the irregularly bedded Fox 

 Hills formation, this irregularity of bedding 

 being due to tidal currents in the shallowing 

 waters of a retreating sea. Above the Fox 

 Hills, and without important break, lies the 

 Lance, which is in the main of fresh-water 

 beds ranging in thickness from 400 to 525 

 feet, and which has a Fort Union-like flora 

 and ceratopsian dinosaurs. Then the marine 

 waters return for the last time in the area of 

 the Great Plains and deposit in the eastern 

 part of the area of the Lance formation the 

 Cannonball marine member, whose fresh- 

 water equivalent is known as the Ludlow 

 lignitic member, the two having each a maxi- 

 mum thickness of about 300 feet. To quote 

 Stanton again : 



The Cannonball marine member rapidly thins 

 toward the west until it is reduced to one or two 

 thin beds which extend as tongues into the pre- 

 dominantly continental deposits of the Ludlow lig- 

 nitic member (p. 9). 



On the other hand, this lignitic member is 

 difficult to distinguish from the overlying 

 Fort Union. Hence we see that there is here 

 a continuous and unbroken series of deposits 

 from the Pierre and Fox Hills into the top of 

 the Fort Union, and that the reported erosion 

 contacts between the several formations are 

 due to nothing more than changes from 

 marine to brackish and fresh-water deposition, 

 or to the irregularities characteristic of con- 

 tinental sediments, the local breaks not repre- 

 senting a loss of geologic time of any marked 

 historical value. 



Finally, Stanton presents two paleogeo- 

 graphic maps, one showing the position of 



the marine Lance with reference to the Pierre 

 sea, and another to the Eocene seas. These 

 maps are a most striking summation of the 

 problem in hand, and at once bring out the 

 fact that the Cannonball member is most 

 closely related to the final Cretaceous seas, 

 since there is no possibility of connecting 

 these marine beds with those of the Eocene of 

 the Gulf of Mexico area, or the Pacific Ocean. 

 Modern stratigraphers know well that it is 

 more commonly the earlier and especially the 

 middle formations of the periods that are pre- 

 served, and that the later parts are more or 

 less absent. As a result of this, the systems 

 of rocks are separated from one another by 

 "breaks," representing intervals of time of 

 varying length, when erosion was going on. 

 No geologist can tell from the stratigraphy or 

 the entombed faunas and floras how long these 

 intervals lasted, but in favorable localities are 

 often foimd sediments not only hundreds but 

 thousands of feet in thickness which on the 

 basis of their contained organic evidence can 

 be shown to supply the record lost elsewhere 

 in one of these breaks. Why is it that the 

 later parts of the systems of strata are absent ? 

 In some cases this is due to lack of deposition, 

 but, as above indicated, in most instances it 

 is because of the inter-system erosion times. 

 The strata last deposited are first to vanish 

 under the influence of erosion, and their 

 absence is almost general in stratigraphy for 

 all the older periods of geologic time. In the 

 more modem periods, however, we should ex- 

 pect the preserval of some of these latest 

 strata of the rock systems, and the Lance with 

 its marine Cannonball member and the over- 

 lying Fort Union appear to be the last 

 deposits of the Cretaceous. They have long 

 been known as the transition formations into 

 the Eocene, but even so they do not fill the 

 entire gap between the Pierre of the Creta- 

 ceous and the Wasatch of the Eocene. As 

 LeConte says: 



But, as the change was gradual and the sedi- 

 mentation continuous, of course the strata were in 

 places conformable throughout. Thus, then, the 

 Cretaceous was before, the Tertiary after, and the 

 Laramie [in which he includes the Lance, Fort 



