COMMlTNKiATION DE M. WARREN Ul'IIAM 
251 
ndia may hâve been coeval with the glaciation of northwestern 
^ urope. In Wales and Yorkshirc the amoiint of dénudation of 
^‘niestone rocks on which drift boulders lie lias been regarded 
Mr. D. Mackintosh as proof that a pcriod of not more lhan 
yO years lias elapsed since the boulders were left in their po- 
®yions. The vertical extent of tins dénudation, averaging about 
inches, is nearly the saine with that observed in the south- 
^est part of the Province of Ouebec by Sir William Logan and 
Robert Bell, where vcins of quartz marked with glacial striæ 
®^and ont to varions heights not excecding one foot above (lie 
"^^athered surface of the enclosing limestone. 
Froin tins widc range of concurrent but independent tcslimo- 
•iies, we may accept it as practically demonstrated that the ice- 
® >eets disappeared only 6000 to 10 000 years ago. Within this 
P®riod are to be comprised the successive stages of man's devel- 
*^P>tient of the arts, from the Urne when his best implements 
''"ere niade of polished stone through the âges of bronze, iron, 
hnally Steel, to the présent time when steel, steam and elec- 
i^ity seern to bring ail nations into close alliance. 
Estimated Duration of tue Ouaternary Era. 
"^rranged in chronologie order, we hâve derived for the three 
of the Ouaternary era, as here defined, the following esti- 
ys of their duration : the Lafayette period or time of pre- 
" cial epeirogenic élévation, with the déposition and érosion of 
® Lafayette beds, soine 60 000 to 120 000 years ; the Glacial 
, regarded as continuons, without interglacial epochs) 
th 'R? ^he culmination of the uplift, but terminating after 
gj^^®^^^®*dence of the glaciated région, 20 000 to 30 000 years ; 
J. the Postglacial or Becent period, extending to the présent 
6000 to 10 000 years. In total, the Quaternary era in 
-^^inerica, therefore, lias comprised probably about 
tlie years, its latest third or fourth part being 
Ole subséquent time. The Tertiary era appears by 
changes of its marine molluscan faunas to hâve been vastly 
having comprised perhaps between two and four million 
> of which the Pliocène period would be a sixth or eighth 
foo-Q exceeding the whole of the ensuing era of great epei- 
ycars 
part 
»cnic movements and resulting glaciation. 
