42 
AMERICAN GEOLOGY, 
according to characteristics which belong and are common to 
each of the respective groups or classes. Attempts were made 
at an early day to construct such an arrangement of rocks as 
would meet the ends in view. These attempts embody the 
views of the prevailing systems of geology, and the nomencla¬ 
ture employed to express the generalizations of the authors 
have been found both defective, as wrnll as expressive of funda¬ 
mental errors. The more recent attempts of classifiers have 
been confined mainly to the use of terms which express facts. 
The rejection of the terms 'primitive , transition , and secondary , 
and perhaps tertiary , seems to be acceded to on the ground 
that they express a theory which is untenable in the light of 
modern discoveries; although the names might continue to be 
employed without endangering the interests of the science, pro¬ 
vided those names were used simply as names, without regard to 
the theoretical views of the authors who first used them. It is 
an interesting fact, that the terms transition, secondary, and 
tertiary, the three periods to which they have been applied, 
stand forth the prominent triads of geologic time. It is no less 
certain that primitive or primary express also truly in the main 
the fact they were originally designed to convey. The nomen¬ 
clatures of all the schemes of arrangement are objectionable, 
inasmuch as they are not consistent with the demands of science. 
The slight modifications which I have proposed in nomenclature 
it is hoped will not be regarded as an unwarrantable innovation, 
as they are the expression of admitted facts. Still there is a 
want of unity in the names, which may be corrected hereafter 
as discoveries are made. The systems into which the hydro¬ 
plastic rocks are divided are arranged chronologically, but the 
names which have been given to these systems are by no means 
chronological. But by dividing these systems into three 
groups, we may express approximately their chronology in the 
terms palaeozoic, mesozoic, and kainozoic. At present the 
most fashionable, and perhaps too the most useful names of 
systems, are taken from localities where those systems are well 
developed, of which we have an eminent example in the word 
