Bibliographical Notice. 475 



feet ; Beaufort beds, 1700 feet ; Koouap beds, 1500 feet ; Karoo 

 shale, or Upper Ecca beds, 1200 feet. 



"Actual Afx'iea, in latitudes north of the Cape, still shows fresh- 

 water lakes surpassing in extent those of other continents ; but 

 the chief lake-basin of the ancient continent, part of which is 

 represented by these reptihferous deposits at the Cape of Good 

 Hope, much exceeded in extent Tanganj-ika, or any of the inland 

 freshwater seas which have been discovered by recent explorers. 



" The attempt to conceive or give intelligible utterajice to tbe 

 sense of past time since the reptiles of South Africa existed in 

 the now upraised lacustrine or estuarine area becomes oppressive ; 

 the terms in which we reckon up phases of the world's history in 

 connexion with human existence are wholly inadequate to convey 

 a clear or comprehensible idea thereof. We are driven, in the en- 

 deavour to realize a conception of prehistoric time, to resort to an 

 artifice akin to that to which the astronomer has been compelled in 

 order to conceive for himself, and convey to others, the relations of 

 space which his instruments and calculus have discovered. 



" The multiplication of millions of miles leaves as vague an idea 

 of remoteness of our planet from the nearest fixed star as the 

 multiplication of millions of years expresses the conviction of the 

 geologist as to the periods needed for the deposit of the thousands 

 of remaining vertical feet of stratified sediment composing even a 

 small subdivision of any of his great natural groups or systems of 

 formations. 



" The ascertained velocity of light yielded the astronomer a more 

 simple, more graspable expression of comparative distances. Light 

 traverses the diameter of the earth's orbit in a quarter of an hour ; 

 it moves at a velocity of 193,000 miles in a second of time. From 

 the nearest fixed star, a Centaicri, a ray of light takes three years to 

 reach the earth ; from Sirius sixteen years. A nebula, near or 

 bright enough to enable the photographer to secure a recognizable 

 image of it, exerts for the purpose a form of force, or sends its 

 ray of light, which probably takes six hundred years in reaching 

 and affecting the instrument and the optic nerve of the human 

 artist. Hence the convenience of defining cosmical spaces or 

 distances in the terms of ' light-years.' 



" So with the geologist : having approximately estimated the 

 period required for the formation of a constituent of the quater- 

 nary, tertiary, or other period, he substitutes, for numerical aggre- 

 gates of historical years, the expression that such or sucli an 

 organism existed and became extinct in such or such a formation — 

 prior, e. g., to the Liassic period, as in the case of the Dicjoiodont 

 reptiles. The name of the formation gives to him an idea of the 

 distance of time since these and other subjects of the present Cata- 

 logue lived, more intelligible than could any row of figures summing 

 up estimates founded on observations of the rates of deposit, of 

 wear, of elevation, or of depression of the several strata of the 

 earth's crust. 



"What the geologist requires in order to receive, in these terms. 



