Geology and Natural History. 218 
face, or surface indented by the solid. It also depends on the 
difference of temperature of the solid and liquid. 
In the case of lead, solid pieces float on the liquid metal, 
although the contraction on solidifying is here marked and well 
known. In fact the solid at 70° has a specific gravity of 11°361, 
while when melted its specific gravity is only 11°07. Floating or 
sinking takes place according to the relation between the volume 
and effective surface ; thin pieces with large surface always float- 
ing, and vice versa.— Wature, 156. E. C. P, 
IL GroLtogy AND NatTurAL History. 
1. Reasons for some of the changes in the subdivisions of Geo- 
logical time in the new edition of Dana’s Manual of Geology ; 
by the Auruor. © 
(1.) Arehawan time.—The first era in geological history is called, 
in the old edition of the Manual, Azozc time or age. The term 
Azoie was always objectionable, because it affirmed what was not 
ved. discovery of the supposed animal fossil called 
Hozoon, shortly after the first edition was issued, led soon to the 
proposed substitution of Hozoie for Azoic. Those who received 
the suggestion with favor did not consider that if the so-called 
Azoic included an Eozoic era, it included a true Azoic also, an era 
of rocks and seas without life; for while the rocks and seas of the 
globe were above the temperature of boiling water, the Eozoic 
era could hardly have begun. The assumption that all those early 
rocks were Eozoic has nothing to favor it. ; 
A general term. for the whole era, free from hypothesis, was 
-sedama needed. Murchison’s term, Bottom rocks, was ripened 
actory. Archean, signifying simply beginning-time, was there- 
fore aes . Und . A shins Re are the Azoic and the 
Eozoic ages, although their limits have not yet been marked out 
in the rocks of the world, and probably never will be, since the 
rocks are now crystalline, through metamorphism, and, with ew 
exceptions, it cannot be learned whether life existed during their 
formation or not. ae a 
I pass now to the Lower Silurian era, which, in the new edition, 
is divided into (1) the Primordial or Cambrian, (2) the Canadian, 
Primo: 
tion, 
(2.) Primordial or Cambrian Period.—This period in the = 
book has unchanged limits, except in the removal of the se - 
€rous sand-rock, the uppermost portion, whose fossils, as stated +f 
Billings and the Geological Reports of Canada, are more are 
related to those of the Tuawing part of the ota Silurian. e 
word Potsdam is dropped because the Po tone is the 
least characteristic cae of the formati term Cambria. 
1s added, because period is identical essentially with the Cam- 
the pe: , “ 
brian of the British geologists. The trilobites and other species 
