CHAPTER V. 
EQUIVALENCY AS INTERPRETED BY GEOLOGISTS. 
DIVERSITY OF INTERPRETATION. 
There is no problem in geology which occasions more controversy 
than that of determining the equivalency between the rocks or forma- 
tions of regions separate from one another. In stratigraphical geol< >gy 
this may be said to be the great problem with which everyone is con- 
cerned until it is settled; and when it is settled it is the one thing 
which every new investigator is wont to think he has a right to criti- 
cise and modify, in the light of his own newly discovered facts. If I 
mistake not, the chief cause for this disagreement regarding geolog- 
ical equivalency is the unconscious confusion of different standards 
of measurement in estimating the values which are balanced, and 
regarding which equality of value is predicated. 
One man, when he speaks of the same formation (e. g., the Medina 
sandstone) as appearing in different States of the Union, is referring 
to the kind of lithological material of which the rock is composed ; it 
is a case of lithological equivalency. Another man is thinking of the 
geological time— the time when the formation was made — in the two 
regions; this is contemporaneity of formations. A third is thinking 
of the likeness of the fossil forms contained in the rocks— fa unal 
equivalency. But in ordinary discussion it is rarely considered thai 
lithological equivalency, contemporaneity of formation, and fauna! 
equivalency are not necessarily the same, and that they may conflict 
with each other. 
In order to make clear the reason for such confusion, the standards 
of equivalency in the case of geography may be examined. In deal- 
ing with geographical facts, there are three ways of measuring and 
defining them. A particular geographical feature may be defined in 
each of three ways. In order to define tin' geographical position of 
West Rock, for example— an elongated hill of trap rock rising to an 
elevation of about 400 feet above tide level— either of the following 
statements may be made: 
1. It is situated about ^ miles north of the head of New Haven Bay, 
on the edge of the New England coast, opposite the central part of 
Long Island Sound. 
2. It is situated in the town of West ville, New Haven County, 
Conn. 
