No. 1.] DAxNA — SOME POINTS IN LITHOLOGY. 43 



felsyte. on account of the obvious external difierences and the 

 wide extent to which the two varieties of rock are distributed 

 over the earth's surface, the epithet "younger" as applied to 

 trachyte and some felsyte 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 

 labradorite-diori/te, called one of the "older," has the same ulti- 

 mate constitution as the augite-bearing rocks, "older" and 

 "younger," called diabase, doleryte and basalt. This fact em- 

 phasizes the great truth, that the rock-making materials of former 

 times are the same as those of recent. 



During and since the Tertiary era more subserial volcanic 

 eruptions have taken place than in any one ancient period ; but 

 there were also many then. As to fundamental differences 

 between the materials ejected by the " older ^' and "younger" 

 world there appear to be none which are of essential importance , 

 Glass or no glass is made an important criterion ; but glass is 

 simply a result of comparatively rapid cooling, and alone indicates 

 no essential differences in the melted mass. 



Dropping the adjectives "younger" and "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 

 beside? 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 than in chemical or mineralogical. 

 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 before, it be- 

 comes, on the " younger " and " older " scale, doleryte or basalt. 



Some of the differences attributed to difference in age may be due 

 to differences in origin — that is, to the rock's being metamorphic 

 in one case, and eruptive in another. There are distinctions of 

 this kind of great interest yet to be followed out ; and they may 

 sometimes have a sufl&cient geological value for recognition in 

 distinct names, although this may not be generally the case. 



2. Foliated or not. — Some rocks are described as having 

 foliated pyroxene or foliated hornblende, that is, diallage, pseudo- 

 hypersthene or smaragdite as the characterizing ingredient. The 

 question here is whether the distinction o^ foliated or not foliated 



