108 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
shell grits and silts (ibid., p. 109, 141) and seldom reaches the thick- 
ness of 200 feet (ibid., p- 133). In some places the bedding is massive 
but elsewhere sands and clays are interstratified with the conglomerate, 
though the stratification is not well marked, and again false bedding 
becomes more common (ibid., p. 182-183). Sometimes the inter- 
stratified clay is in lenticular layers or even in balls (ibid., p. 198). 
Variations in thickness and in composition along the dip of the sev- 
eral beds are much greater than along the strike and all the beds 
are more or less lens-shaped in cross-section, first thickening and 
then ,thinning seaward (ibid., p. 370). 
The extent of the Basal Sands is a noteworthy feature. ‘They were 
deposited successively farther inland during the transgression of the 
sea and hence underlie beds of varying age from older to younger, 
into which they pass horizontally as indicated in Figure 1, taken from 
Paleozoic 
Fig. 1.— Diagrammatic section of the Basal Sands of the Cretaceous of Texas, 
showing the normal arrangement of sediments with an encroaching sea 
(slightly modified from R. T. Hill, p. 202). 
Hill’s description (ibid., p. 202). They thus have great extent yet 
throughout they maintain a remarkable. similarity and uniformity 
in lithological nature (ibid., p. 179). 
The Cretaceous system of Texas rests unconformably upon the 
planed off edges of different layers of Palaeozoic rocks that had en- 
dured a long period of erosion (shown by the absence of ‘Triassic and 
Jurassic sediments) and had probably been reduced nearly to base - 
level. . The ancient land surface of that time has been termed by Hill 
the “Wichita paleoplain.” Irregularities of configuration of this 
Precretaceous land are shown by some degraded remnants that still 
persist, like the Ouachita Mountains (ibid., p. 363). 
