MANSFIELD: ROXBURY CONGLOMERATE. 107 
This chapter is concerned chiefly with the characteristics of true 
conglomerates as determined by their modes of origin but it includes 
a discussion of certain rocks that have been named and described as 
conglomerates, although in.the light of the above definition they would 
more properly be classed under another heading (see, for example, 
the Dwyka Conglomerate, p. 130). 
TYPES OF CONGLOMERATE.— The principal types of conglomerate 
that will receive consideration are: (1) marine; (2) fluviatile; (3) estua- 
rine; (4) lacustrine; (5) glacial. To these may be added the pseudo- 
conglomeratic rocks produced by internal movements of the strata 
and called (6) crush-conglomerates. Moreover, (7) it should be 
remembered that many combinations of processes are possible and 
that many accumulations doubtless owe their origin to such combin- 
ations. 
Marine Conglomerates: — Cretaceous of Texas. One of the best 
examples of marine strata is the Cretaceous system of Texas, so 
well described by R. T. Hill. A persistent feature of this system 
is the formation known as the Basement or Trinity Sands. They 
are said (R. T. Hill, p. 132) to be usually accompaniéd at the 
base by a fine pebble conglomerate. At one of the places men- 
tioned (ibid., p. 141), where the basal beds are locally known as the 
Sycamore Sands, the matrix of the conglomerate is described as a 
“cement of ferruginous, yellow and red, gritty sand.” The material 
of the pebbles in the conglomerate varies locally according to the 
nature of the adjacent, pre-existing rocks (ibid., p. 132) and may 
.consist of masses of limestone, quartz, chert, granite, and schist, four 
to six inches in diameter at the base and decreasing in size upward. 
They are well rounded at the locality above mentioned (ibid., p. 141) 
but elsewhere (ibid., p. 181-182) many of the cobblestones are de- 
scribed as but slightly worn. In the latter case the basal conglomerate 
consists chiefly of limestone, sandstone, and chert of local derivation, 
while quartz pebbles from more remote sources are smaller and well 
rounded. The conglomerate bed exposed in Pompey and Blanket 
Creeks (Brown County) is described as furnishing a fine illustration 
of false bedding (ibid., p. 183). 
The Basement Sands contain thin beds and laminae of red and blue 
clay (ibid., p. 132). The Denison beds of the Washita division also 
contain ferruginous clays and as a whole they are said to be “charac- 
terized by the strong ferruginous colors peculiar to near-shore deposits” 
(ibid., p. 266). 
The basal conglomerate grades upward into sands, false-bedded 
