AGE OF THE EARTH—JOLY. O77 
obviously, give us no information as to the relative durations of the 
geological epochs. The latter question can be approached in two 
ways. (1) By means of the stratigraphical column or measured 
maxima of detrital and chemical deposits, assuming that these were 
laid down at an approximately uniform rate; and (2) by the radio- 
active method. TI shall first consider the former method. 
THE AGE FROM THE SEDIMENTARY COLUMN. 
As the result of the observations of geologists in many parts of the 
world, the maximum thickness of the strata deposited in the various 
geological periods may be estimated as follows: 
Feet 
iRecentand /Pleistocenetase P48 a ss ee at 4,000 
Plioceiga. 24.3.0! 4: aad: sh aceeetgatasenheceadis: - - 13, 000 
IMO CET Cistercian wa on ae oy ee pre 2 8 14, 000 
(POe eNO shes awe emer Meee tam aes atta bio ous eeils 12, 000 
EN OYGCESAVE)S See Ses cr tc a ae ie 20, 000 
——— 63, 000 
Upper Cretaceousi Othe RI aA IE SL 24, 000 
Lomon Crotaceous= $425.0 cde set aed Eh. -geene hese 20, 000 
ANDRES 1 ae ee ee eye ee eee 5 See 8, 000 
FIRE Le NE ee ee ee An aloes Gene ee a oe aera ae ae 17,000 . 
69, 000 
52) 5000: 1s) WA CA oe LEAS UES GC Mee Seay Se Leta eae ale ee 12, 000 
Carbonilerouss: a4 j346.e 5 ee or Oger. Skee ees bo 29, 000 
IDG Vom amet es tae eS cel a yee ee Be. Ps 22, 000 
63, 000 
SIGS oy ae eevee ist tes Aah pain ERD 15, 000 
Ord Ovi Cianeeteeee seen ee eee mee ci ce ae eee coe omiets 17, 000 
Cerna riety RES ee Ed EE SEE GES be es OTIS 2) ot) Ue 26, 000 
58, 000 
CG WCENS Walle oo hae = aa ote Sos ced oie ee 50, 000 
ACTUMIN ed Me ce ye ee ste eee tae ees erst ee epee ees 14, 000 
EPEAT ee oe at Soe one ome cis .. 18,000 
82, 000 
AGI SeARNERTES EEE 7 = PORTE SEIU ES OT). SOME STR CE I Ee fF ? 
NOtaled sdb dee ® oisd Loker. moe ce Sees BL ee 335, 006 
This compilation is due to Prof. Sollas. 
It is not probable that there will be in the future any very large 
amendment of these figures so far as they refer to post-Algonkian 
time. The Jurassic, as Sollas observes, seems deficient. The pre- 
Cambrian is the most obscure among the estimates. It claims our 
special attention, not only with reference to the thickness of accu- 
mulated sediments, but in so far as the observations may throw 
light on the denudative conditions of the time. 
In no part of the world are pre-Cambrian rocks better developed 
and exposed than in and around the Archean shield of Canada; and 
1 Presidential Address, Geological Society, London, 1909. 
