MICROSCOPICAL PETROGRAPHY 483 
be done over later on. A mechanic who attempts to make a fine 
spirit level or microscope with poor tools cannot produce a good 
job. A petrologist on detailed work who describes rocks in the 
way they were described thirty years ago, and uses the micro- 
scope as it was then used, is doing an injustice, not only to the 
reader, but to the microscope and the tools of precision which are 
now placed at his disposal. 
There is a genuine pleasure on the other hand, in making the 
most out of the tools we have, and in gathering data, the accuracy 
of which we know definitely and can state in terms which will be 
intelligible to observers a century hence. In short, it is only by 
the accumulation of tangible facts, and grouping of such facts by 
correct generalization, that real progress is made. From a few 
facts a mind of genius may intuitively infer and state a correct 
generalization which covers a whole group of facts to be discovered 
later, but most of us are not in that class, and it is our duty to 
assemble the facts—facts which are real facts, based on precise 
data of observation. 
Strictly speaking, quantitative work means control over all 
parts of a given system. The order of accuracy of all measure- 
ments made is definitely known and any observer at a later date 
should be able to repeat the measurements and obtain similar 
results. The system is, in short, reproducible. Quantitative 
work is tangible throughout and we know definitely its behavior 
at all times during our observations. In qualitative work, on the 
other hand, the system is imperfectly defined, no definite limits 
are set, and the results, obtained, lack precision; their probable 
error is unknown and they are indefinite and uncertain to that 
extent. A strictly quantitative piece of work, the accuracy of 
which is adequately stated, produces on the observer, and on the 
reader as well, a feeling of confidence and stability, which quali- 
tative work with its uncertain elements can never produce. A 
sense of control and mastery over the factors of an intricate system 
is a natural sequence of good quantitative work and is, psycho- 
logically, one of the greatest rewards granted to the student of 
nature. 
There is still another feature which may be emphasized. In 
