THE AECHEAN. 161 
Prof. J. L. Le Conte answers that " to classify the eruptives 
occurring in the Archean with the Archean as of Archean age 
would, it seems to me, lead often to errors, as so much Archean 
is exposed only by erosion and the eruptives may be much 
younger." In fact, he thinks we must study eruptives as eruptives 
only — until we understand them thoroughly, and then we may 
or may not be able to correlate kinds with age. At present 
he does not believe in any classification of eruptives by age. 
Prof. Roland D. Irving recalls that there are two classes of 
eruptives found in the Archean : one erupted in Archean time, 
and the other subsequently. Sometimes these classes can be dis- 
tinguished, but as a rule they cannot. In any sort of a general 
map of a region he does not think that the Archean eruptives 
should be distinguished from the rest of the Archean. In local 
mapping, however, when the eruptive origin of certain granitic 
areas has been clearly established, then he would map them sepa- 
rately. Similarly he would map special areas of basic eruptives 
which have been clearly proved to be such. But in both these 
cases he would map the rocks under their special names — such as 
"granite," "diabase," or whatever it might be; and not simply 
as "eruptive;" for a considerable portion of the Archean can, he 
thinks, be clearly shown to be of eruptive origin. That is^ 
schistose structure has been superinduced upon both basic and 
acid eruptives. As to the origin of a still larger proportion of 
the schistose Archean, he thinks we know nothing. Therefore, 
to set forth only portions of the Archean as "eruptive," without 
specifying the kind of rock, raises the presumption that the 
remainder is non-eruptive — a presumption that would certainly 
be, in a measure, false. 
Mr. Arnold Hague is of the opinion that rocks fjdund in 
the Archean, the eruptive nature of which is unquestioned, should 
be classed independently of the sedimentary and metamorphosed 
strata. 
Dr. S. F. Emmons thinks that the Archean eruptives should 
be treated in the same manner as those of other geological sys- 
tems. It is not always possible to determine whether they were 
erupted in Pre-Cambrian times or not ; and even if it were, the 
color scale is not extended enough to furnish distinct colors for 
the eruptives of each system. 
