482 FRED, EUGENE WRIGHT 
visits a new country, like Alaska, to obtain a general idea of the 
geologic lay of the land and the rock types there represented. 
During this reconnaissance period, petrography, or rock descrip- 
tion, was the prominent feature of petrology, as the thick papers 
of that time testify. Men were interested in rock types and 
rock classification. They wished to cover the entire field, and to 
do so had, of necessity, to adopt the methods of reconnaissance. 
Their methods and their classifications were all essentially quali- 
tative in nature. After this preliminary work came the more 
detailed investigations, such as are represented in geology by folio 
and economic work with large-scale base-maps and only limited 
areas to cover in a given time. 
A science must always develop from the qualitative to the 
quantitative, and the process is necessarily a gradual one. In 
science the term qualitative is usually applied to statements in 
which no definite limits to the quantities involved are expressed, 
while in quantitative statements such limits are definitely set. 
These limits may vary widely in their order of magnitude and 
one quantitative statement may be only roughly quantitative 
(first approximation) while a second may be highly precise. 
No observations are ever absolutely accurate; the absolute quan- 
titative cannot be attained in the physical world and the idea 
of limits or degrees of approximation to truth (probable error) 
pervades all science. Such limits establish at once boundary lines 
or fences within which speculation must be held. In a qualitative 
statement such limits are not given, with the result that they 
may be arbitrarily extended or decreased by the investigator as 
the exigencies of his case demand. The smaller the limits in 
quantitative statement the higher the degree of approximation to 
truth, the fewer the possibilities for misinterpretation, and the 
greater the probabilities for correct generalization by the scientist. 
The growth of a science rests, in part at least, on the development 
of exact methods of attack and on the precise data of measurement 
accumulated thereby. 
The observer who applies only reconnaissance methods to 
detailed work requiring exact methods is doing an injustice to the 
work, and is actually wasting his time, because such work has to 
