§ 42. RETROSPECT. 



831 



assemblage of sedimentary strata. In the present case the 

 outer or overlying gneiss and mica-schist that envelope 

 the lower region of the mountain are the most ancient ; the 

 granite is the next in age, having protruded through, and 

 upheaved the gneiss ; and the central nucleus of porphyry 

 is the youngest, or last erupted rock, having been forced 

 up through the dome of granite. These three phases of 

 plutonic action may have taken place at different and very 

 distant periods ; in like manner as the beds of tuff and 

 scoriae of Vesuvius or Etna, ejected a thousand years ago, 

 may be upheaved and traversed by the modern eruptions 

 of incandescent lava. 



42. Retrospect. — I now approach the termination of 

 this argument, and it will be instructive to review the 

 phenomena which have passed before us, in order that we 

 may retain a clear conception of the leading principles and 

 inferences that have been enunciated. I shall, therefore, 

 in the first place, offer a summary of the most important 

 changes which have taken place in the animal and vege- 

 table kingdoms, and in the physical conditions of the 

 earth's surface, during the vast periods which our investi- 

 gations have embraced ; and conclude with a retrospective 

 survey of the effects of vital action in the elaboration of the 

 solid materials of the crust of the globe. 



With the view of recalling the principal facts, I now place 

 before you the series of Illustrations employed in these 

 Lectures, that you may perceive at a glance the striking 

 contrast presented by the Faunas and Floras of the respec- 

 tive geological epochs.* In the first stage, traces of the 

 existing species of animated nature were everywhere appa- 



* The reader may realize this idea by referring to the illustrations 

 of these volumes, commencing with the fossil human skeleton (ante, 

 p. 88), and proceeding from the large mammalia (pp. 151, 176), to the 

 last of the series, the fossils of the palaeozoic deposits (pp. 736, 761, 

 790). 



