No. 1.] DANA — SOME POINTS IN LITHOLOGY. 41 



In the following pages I propose to consider the value of some 

 of the distinctive characters which are generally accepted at the 

 present time in defining certain kinds of rocks. 



1. ^^OMer'' and ^^ younger.'' — The distinctions "older" and 

 ^' younger " often applied to a number of kinds of eruptive rocks, 

 seem to imply that the earth has generated different kinds of 

 rocks as it has grown old. The terms have reference, however, 

 to only one epoch of abrupt change — that between the cretaceous 

 and tertiary, " older " signifying pre-Tertiary, and "younger" 

 Tertiary or later in date. It is of eminent importance to geology 

 to know definitely whether this epoch was one of great change 

 in the earth's ejections, and an epoch so marked that the rocks 

 on one side of the time-boundary are deserving generally of 

 different names from those of the other; for thus lithology, 

 judging from some recent works, as well as older, has seemingly 

 decided. Some examples of the "older" kinds are dloryte^ 

 diabase, and a large part offehyte ; and some of the "younger" 

 are propyJyte, doleryte or basalt, and trachyte. The value of 

 (the distinction may be learned from a comparison of the rocks 

 of one of these series with the rocks of the other. 



First as to diabase and doleryte. Typical diabase consists 

 according to the descriptions, of labradorite and augite, with 

 some magnetite or titanic iron ; and so does doleryte. Diabase, 

 to a large extent, is a crystalline-granular rock, so is doleryte. 

 Diabase was formerly supposed to be peculiar in containing 

 chlorite, but it is now proved, as asserted by Rosenbusch, that 

 chlorite is not an essential characteristic, so that diabase may be 

 chloritic or not ; and the same is true of doleryte. Old diabase 

 was described as differing from the younger rock doleryte in 

 containing no glassy portions or grains among the crystalline 

 grains ; but this is also set aside by later observations, and Rosen- 

 busch accordingly divides diabase into (1) massive granular 

 diabase, (2) diabase-porphyrite, and (3) glass-bearing diabase; 

 and corresponding subdivisions are as good for doleryte. Thus 

 in chemical composition, in mineral composition, in texture, in 

 the presence or absence of chlorite, in the presence or absence of 

 glassy portions, the two rocks are identical. Analyses of " dia- 

 bases " from the Archaean to the Tertiary, and of " dolerytes " of 

 subsequent time, have shown that material of essentially the 

 same composition, has been ejected in all geological ages, as has 

 been well urged by Allport and others. The analyses might be 



