14 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



conditions. Anyone familiar with the sudden appearance of an abundant 

 vegetation in an arid region can realize how a single season of exceptional 

 rainfall might furnish enough carbonaceous material to radically change the 

 color of a goodly thickness of deposits. The subaerial deposits of the 

 Permian and Triassic are notably lacking in persistence, either as to bedding, 

 material, or color. Williston and Case have asserted their conviction that 

 sections made at any point in these beds can not, in most cases, be depended 

 upon a quarter of a mile away. 



II. GEOGRAPHICAL LIMITS OF THE UNIT. 



a. Mapping of the limits of the unit, preferably upon a topographic base, 

 is a primary essential. Most commonly any exact determination of the 

 outline is difficult or impossible because of the burial of a part of the unit 

 under younger beds, because of the destruction of a part of the bed by 

 erosion, or because of its interruption by structural changes. The location 

 of hypothetical limits is always a most uncertain process and demands 

 the utmost care and the use of every possible check, such as the physiography 

 of the underlying beds, recognition of the horizontal changes in the material, 

 identification of outliers, interpretation of well records, correlation with 

 other outcrops, etc. 



b. Location of the source of the material. As suggested above, the location 

 of the inner line of the deposit and the origin of the material is of the utmost 

 value. If the deposits are largely or even in part clastic in character, a 

 knowledge of the source will give much information as to the course and direc- 

 tion of the transporting currents, the distance covered in transportation, 

 and the weathering or other changes that the material has undergone. At 

 the same time, an idea may be gained of the physiography of the old land. 1 



In the case of a transgressing sea with progressive overlap 2 of the beds, 

 new material will be constantly gained by the waves and mingled with 

 material carried in by rivers. The basal beds will be likely to be conglomer- 

 ates, especially if the sea is advancing against a resistant coast, and this 

 conglomerate will be a useful and easily followed criterion in determining 

 the limits of the beds. In a regressing or stationary sea the upper beds 

 will be finer as the waves work over the already deposited material or dis- 

 tribute the material carried in by rivers. Great flats would be formed as 

 the sea retreated from its shelf and the streams poured out their waste. If, 

 however, the retreat of the sea were rapid, caused by a sudden uplift of the 

 land, the quickened streams might carry out much coarse sediment which, 

 reworked by the tidal flux and by storm waves, would need careful observa- 

 tion of its character and content, petrographic and fossil, to distinguish it 



1 Barrell, Jos., Relation Between Climate and Terrestrial Deposits, Jour. Geol., vol. xvi, 



Nos. 2, 3, and 4, 1908. 



2 Grabau, Types of Sedimentary Overlap, Bull. Geol. Soc. Amer., vol. 17, p. 567, 1906. 



