338 J. D. Dana on some points in Lithology. 
wide extent to which the two varieties of rock are distributed 
over the earth’s surface, the epithet ‘‘ younger” as — to 
trachyte and some felsy te can subserve plainly no good use. 
The essential chemical identity of the “older” and “ younger” 
rocks is further exhibited in the fact that the hornblende- 
bearing rock lisboa aie called one of the ‘ older,” has 
the same ultimate constitution as the augite-bearing rocks 
‘“‘older” and “younger,” called diabase, doleryte and basalt. 
This fact emphasizes the great truth, that the rock-making 
materials of former time are the same as those of recent. 
During and since the Tertiary era more true subaerial vol- 
canic eruptions have taken fia than in any one ancient 
eriod ; but there were also m then. As to fundamental 
differences between the (aaderiate ejected by the “older” and 
‘‘vounger” world baie appear to be none which are of essential 
importance. Glass or no glass is made an important criterion ; 
but glass is simply a Soils of comparatively ra cooling and 
alone indicates no essential differences in the melted mass. 
Dropping the adjectives “ younger” seit “ older” would 
require the dropping of the distinctive names based on them, 
unless some better reason exists for retaining them. 
If diabase is not distinct from doleryte in some important way 
besides that of time of eruption, the name diabase (the newer 
of the two) is unnecessary. In fact, the rocks are not distinct 
in external characters any more ht in chemical or miner- 
alogical. The rock of the Giant's Causeway was pronounced 
diabase on microscopic grounds when its geological age was 
unknown ; but it has since been proved to be Miocene Tertiary ; 
and now although just as much diabase in constitution as be- 
a it becomes, on the “younger” and “older” scale, doleryte 
or 
Some ee the differences attributed to difference in age may 
be due to differences in origin—that is, to the rock’s being 
is of ores y importance to be used as a distinction among 
kinds of rocks. 
In the first place it is trivial as a crystallographic distinction. 
Secondly, although mineralogy once made ea 3 of the distine- 
