The Jurassic Horizon. — Keyes. 291 
and Hayden). It rests unconformably on all older forma- 
tions, from the Mid-Carboniferous limestones to the 
Comanche series. 
The shale (b) beneath the massive sandstone at the top 
of the Tucumcari section, has been correlated with - the top- 
most portion of the Washita division of the early Cretaceous 
as represented farther to the eastward in Texas. The lower 
members of the early Cretaceous successively thin out to' 
the north and west from the central part of that state and 
each overlaps the next below. 
The next formation below (a), which rests unconform- 
ably upon the Triassic Red Beds and which has been par- 
alleled with the Trinity sands of central Texas appears to 
be a formation with as yet no tangibly determinable rela- 
tionships. It may be the basal member of the earl}- Cre- 
taceous section which is so well developed farther to the 
eastward, and this has been the view advanced by Hill and 
other workers in the Texas field. Or, it may be a littoral 
deposit that followed up an advancing- shore; and thus it 
may have an age in its different parts extending throughout 
the Comanche period. However, this phase of the subject 
receives full discussion elsewhere. 
This so-called Trinity sandstone stands apart from all 
else. It has no direct genetic relationships with the forma- 
tions either above or below. The unconformable relations 
that it bears both to the superior strata and to the inferior 
rocks clearly indicates the lapse of a considerable time in- 
terval at least at the base. There is then in this sandstone 
an important geological formation about which little is yet 
definitely known regarding its geological relationships; and 
to the westward at the same horizon an important erosion 
interval is represented. The equivalents of these in the 
sediments of other localities, as well as the space they repre- 
sent in the general geological column have become topics of 
some speculation. 
White, who was long the best American authority on 
the Cretaceous rocks, considered the Trinity sands, or 
Dinosaur beds, as reaching down into the Jurassic period. 
Marcou regarded the Jurassic as partially represented by 
this formation. Most writers have disputed the existence 
