908 



THE WONDERS OF GEOLOGY. Vol. II. 



steps, as the barrier was worn away, the lowermost terrace was at 

 length formed; and ultimately the system of lakes disappeared, from 

 the barrier having been entirely removed. 



E. Page 892.— Sir John Herschel on the possible variability op 

 solar inpluence, (ante, 742 — 745.) — Since those pages were printed, 

 another astronomical source of periodical variability in the general tem- 

 perature of the earth's surface, has been suggested by Sir John Herschel, 

 in his magnificent recent work, entitled, Results of Astronomical Obser- 

 vations made at the Cape of Good Hope. Instances of variability in 

 the splendour and brightness of certain stars have long been familiar 

 to astronomers ; and some remarkable phenomena of this kind were 

 observed by Sir J. Herschel at the Cape ; as, for example, in the star 

 7] Argus, whose light became tripled in brightness in the course of 

 three or four weeks, and then faded to its former appearance. Con- 

 siderable variability in the luminosity of some of the stars of Ursa- 

 Minor has also been observed of late years. From these facts Sir J. 

 Herschel suggests the possibility that the sun of our system may in 

 the course of ages be subject to similar phases of augmented or 

 diminished energy ; and that such variability in the periods contem- 

 plated by Geology, may have given rise at one epoch to a general 

 equatorial climate, and at another to one far below the general tem- 

 perature that now prevails. "The grand phenomena of Geology 

 afford, as it appears to me, the highest presumptive evidence of changes 

 in the general climate of our globe. In the slow secular variations of 

 our supply of light and heat from the sun, which, in the immensity 

 of time past may have gone to any extent, and succeeded each other 

 in any order, without violating the analogy of sidereal phenomena 

 which we know to have taken place, we have a cause, not indeed 

 established as a fact, but readily admissible as something beyond a 

 bare possibility, fully adequate to the utmost requirements of geology. 

 A change of half the magnitude in the lustre of the sun, regarded as a 

 fixed star, spread over successive geological periods — now progressive, 

 now receding, now stationary, according to the evidence of warmer or 

 colder general temperatures which geological research has disclosed, 

 or may hereafter reveal — is what no astronomer would now hesitate 

 to admit as in itself a perfectly reasonable and not improbable suppo- 

 sition. Nor can it be objected that the character of a vera causa is 

 wanting in such a hypothesis." 



F. Page 894. — Mr. Darwin, on the Galapagos Archipelago. — 

 " This archipelago consists of ten principal islands, of which five 

 exceed the others in size. The largest, Albemarle Island, is of an 

 angular form, and 100 miles in length. They are all formed of 

 volcanic rocks; a few fragments of granite, curiously glazed and 

 altered by heat, can scarcely be considered as an exception. Some of 





