118 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
3. It is situated on the meridian of 41° 20' north latitude and on the 
parallel of 72° 57'+ west longitude. 
From this illustration it is evident that any geographical feature on 
the face of the earth may be defined as to its geographical position in 
three distinct ways — distinct, because the locality scale may be any 
one of the three kinds signified in the foregoing definitions. 
These three locality scales are : 
1. A geographical locality scale, in which the facts are the present 
configuration of the surface of the earth, chiefly in respect to alti- 
tude, or distance in feet above or below sea level. 
2. A political locality scale, in which the facts are the political 
divisions of territory as defined by human ownership or occupation. 
3. An astronomical locality scale, in which the facts are distances 
in angular degrees or minutes, north or south from the equator of 
the earth and east or west from an arbitrary standard meridian (that 
of Greenwich). 
It will be observed that the only one of these standard scales which 
is permanent, fixed, and capable of use with precision is the astro- 
nomical scale, which can not be seen on the surface and has no regard 
whatever to facts upon which the other two scales are constructed. 
I have referred to the locality scales of geograph}' in order to illus- 
trate more vividly the differences which are confused when a time- 
scale is under consideration for the definition of geological facts. 
The geologist is using three time-scales in his attempt to define the 
chronological relations of geological events. 
1. When an American geologist speaks of a formation in Ohio as 
the Trenton limestone, or in the Appalachian region speaks of the 
.Medina sandstone, or the Catskill, or Poeono, he is using a time-scale 
in which the basis of classification is the fact that a rock of a particular 
kind in the section at Trenton, Medina, or in the Catskill or Poeono 
Mountains is assigned to a definite place in the stratigraphical 
sequence of formations. In applying the name to a formation in Ohio 
or in the Appalachians, he is attempting to affirm equivalency of posi- 
tion in a stratigraphical series of formations. It is a time classification 
by formations; he is dealing with a formational time-scale. 
2. Again, in describing the Niagara of America as equivalent to the 
Wenlock, and then classifying it as therefore belonging to the Silurian 
age, the geologist is using an entirely distinct basis of classification. 
The basis of his determination now is equivalency of the faunal combi- 
nation of fossil species found in the rocks of the two formations. In 
this case stratigraphical or lithological characters are not in evidence, 
but only the organisms which were living when the sediments com- 
posing the rocks were laid down. It is now a faunal time-scale. 
3. There is still a third method of defining geological events chrono- 
logically. The question arises in mapping the rocks of a region, where, 
in the column of formations, shall the boundary be drawn between 
two systems, viz, between the Silurian and Devonian? This question 
