Williams] NOMENCLATURE OF FORMATIONS. 27 
of Washington in 1901. a In this paper definitions tending to clarify 
thinking in these directions are given. In another paper, b read 
before the Connecticut Academy, February 12, 1002, a brief synopsis 
of the results of the investigations given at length in this bulletin 
are stated, and some laws not specifically formulated in this paper 
are there given. 
In order to call attention to the distinctions which are made by a 
separation of the discussion of fossil faunas from that of the geolog- 
ical formations in which record of them is preserved, it may prove 
useful to mention in this place the terms in common use as well as 
those here introduced, classified according to their application to 
formations or faunas. 
NOMENCLATURE OF FORMATIONS. 
Formations are portions of the rocky crust of the globe. They may 
be called igneous, sedimentary, or metamorphic, according to their 
mode of origin. They may receive lithological names, as granite, lime- 
stone, or sandstone, according to their lithological constitution. 
The terms sheets, intrusive or extrusive strata, lenses or lentils, 
apply to formations on the basis of their geological structure. 
They are called crystalline, schistose, stratified, or oolitic, on the 
basis of their texture. 
They are described and mapped as occupying particular geographical 
areas on the basis of their present outcroppings to the surface of the 
earth. Their thickness is determined by measuring them from bot- 
tom to top in a line vertical to the plane of their supposed original 
deposition, and they are said to be older or younger according to their 
order of succession. 
They are named on the basis of their local, prominent;, or first - 
described geographical outcrops. These names are generally geo- 
graphical terms. 
They are classified primarily on the basis of their observed order of 
succession, and secondarily on the basis of their supposed equiva- 
lence in stratigraphical position with other formations whose order 
of succession has been established. Such terms as system, series, 
groups, stages, zones, and beds are thus applied to geological for- 
mations; station, section, geological column, outcrop, conformity and 
unconformity, province, region, and like terms also apply to geological 
formations. 
The terms correlation, contemporaneity, and equivalency apply to 
formations, and may be used on the basis of structural, lithological, 
or stratigraphical evidence; but in general it is only on the basis of 
evidence furnished by the fossils within them that they become widely 
applicable. 
«The discrimination of time values in geology: Jour. Geol., Vol. IX, pp. .")7<> 585. 
''Fossil faunas and their use in correlating geological formations: Am. Jour. Sri.. 4th series, 
Vol. XIII, pp. 417-432. 
