CHAPTER XXV. 
On the rocks usually termed £ Primary ' — Their relation to volcanic and sedimen- 
tary formations — The ' primary' class divisible into stratified and unstratified 
— Unstratified rocks called Plutonic — Granite veins — Their various forms and 
mineral composition — Proofs of their igneous origin — Granites of the same 
character produced at successive eras — Some of these newer than certain fos- 
siliferous strata — Difficulty of determining the age of particular granites — 
Distinction between the volcanic and the plutonic rocks — Trappean rocks not 
separable from the volcanic — Passage from trap into granite — Theory of the 
origin of granite at every period from the earliest to the most recent. 
ON THE ROCKS COMMONLY CALLED PRIMARY. 
We shall now treat of the class of rocks usually termed c pri- 
mary,' a name which, as we shall afterwards show, is not always 
applicable, since the formations so designated sometimes belong 
to different epochs, and are not, in every case, more ancient 
than the secondary strata. In general, however, this division 
of rocks may justly be regarded as of higher antiquity than 
the oldest secondary groups before described, and they may, 
therefore, with propriety be spoken of in these concluding 
chapters, for we have hitherto proceeded in our retrospective 
survey of geological monuments from the newer to those of 
more ancient date. 
In order to explain to the reader the relation which we con- 
ceive the rocks termed e primary ' to bear to the tertiary and 
secondary formations, we shall resume that general view of the 
component parts of the earth's crust of which we gave a slight 
sketch in the preliminary division of our subject in the 
2nd chapter *. 
We there stated that sedimentary formations, containing 
organic remains, occupy a large part of the surface of our con- 
tinents, but that here and there volcanic rocks occur, breaking 
through, alternating with, or covering the sedimentary deposits, 
* See above, p. 8. 
