wili.tams] EQUIVALENCY AS INTERPRETED BY GEOLOGISTS. 119 
was settled in the case of the Appalachian sheets of the U. S. Geolog- 
ical Survey by drawing the line in the midst of the Monterey sandstone. 
In the legend of the map the Monterey sandstone is called neither 
Silurian nor Devonian, but transitional. In the text, the forma- 
tion is defined as containing Oriskany fossils. Without entering 
into the merits of the case, this is an illustration of using a scale which 
is neither f ormational nor faunal. A formation is a distinct lithological 
unit, but its base, as thus defined, is placed below the boundary line 
between the two systems, and its top is above the boundary line. This 
boundary line is therefore a theoretical one, which does not occur in 
the stratigraphical series as mapped on the sheet, and the scale to 
which it is referred is the standard geological time-scale. 
This particular standard is based upon a single section in Wales, 
where the earliest recognized boundary was drawn between the Silu- 
rian and overlying Old Red sandstone, and however differently the 
sequence of formations or faunas may occur in any other regions, the 
grand divisions of time — Cambrian, Silurian, Devonian, Carboniferous, 
etc. — are arbitrarily drawn, determined as near as may be by compar- 
ing all the points of geological history for the two separate regions. 
In the previous pages the facts are presented by which the applica- 
tion of the rules for establishing equivalency may be illustrated. 
In the case of the Devonian formations and faunas of the New York 
province the different kinds of equivalency may be stated with some 
degree of precision. 
In a formational time scale the units compared are lithological units. 
Examples of such units are the black Genesee shale, the Huron 
shales of Ohio, the Tully limestone, the Catskill, the Oneonta, and 
the Hamilton formations. The questions of formational equivalency 
involve two points — lithological and stratigraphical equivalency. 
In two neighboring sections there may occur 50 feet of red sandstones 
in one, which are equivalent to 75 feet of red sandstone in the other 
section ; this is a case of lithological equivalency. In two other sections 
50 feet of red sandstones in one may be equivalent to 30 feet of green- 
ish shales and flags in the other; this is a case of stratigraphical equiva- 
lency. From the examples given in discussing the Devonian faunas 
it is evident that the lithological and stratigraphical equivalency may 
coincide or may be discordant. 
In ordinary cases it is presumed that lithological and stratigraphical 
equivalency coincide. Such is the case when the Tully limestone is 
followed along the line of its outcrops. When its calcareous character 
becomes so faint as to be indistinguishable in the series of si rata, the 
formation is said to cease. According to the older habits of treatment 
of such cases the Tully limestone is supposed to thin and run out to 
a feather edge, thus finding its equivalency in the column between 
the subjacent and superi m posed formations. According to the inter- 
pretation here proposed the change would be described as a. Lithological 
change— a change in the character of the sediments by increaseof the 
