THE CAMBRIAN PERIOD. 275 



ment is known to accumulate, this long period cannot be reduced to 

 years. It is quite certain that the rates vary greatly. Besides many 

 obvious illustrations of the minor sort, there is this very general and 

 important one: When the land is relatively broad and high, and while 

 the deposition belt around its borders is narrow, as is apt to be the 

 case after a period of continental protrusion, the amount of material 

 eroded in a given time is likely to be large, while its concentration on 

 the limited deposition area necessarily gives great proportional thick- 

 ness and rapidity of rate to the accumulation. On the other hand, 

 when the continent is approaching base-level, and the land is low and 

 limited by the encroachment of the epicontinental seas, and while these 

 seas are wide and shallow, and subject to agitation over broad tracts, 

 the amount of material eroded in a given time is relatively small, while 

 the area over which it is spread is exceptionally large, and hence it adds 

 but slightly and slowly to the thickness of the deposits. In Washington 

 County, N. Y., there is a bed of Cambrian limestone 1400 feet thick 

 beneath 10,000 feet or more of fragmental rock. It has been estimated 

 that limestone sometimes forms at some such rate as one foot per 

 century. 1 At this rate this limestone alone would have required 

 140,000 years. If the overlying fragmental rocks required an equal 

 length of time for their accumulation, the total length of the period 

 would be 280,000 years. In some parts of the West there are 6000 feet 

 of limestone besides thick bodies of fragmental rock. At the same rate 

 of accumulation, the 6000 feet of limestone would call for a period of 

 600,000 years, and if time be allowed for the other formations of the 

 same region, the period would be greatly lengthened. It should be remem- 

 bered, however, that while one foot per century may be a rate at which 

 limestone sometimes accumulates, it does not follow that it is the rate 

 at which the Cambrian limestones were formed. The data on which 

 this estimated rate is based are believed to give too high, rather than too 

 low a rate, and a less rapid accumulation would mean a correspondingly 

 longer period of time. 



Many estimates of geological time, based on various data, have 

 been attempted. 2 These estimates, so far as applied to the Cambrian ; 

 generally assign to that period a duration of 1,000,000 to 3,000,000 years. 

 It should be distinctly borne in mind, however, that the chief value 



1 LcConte, Am. Jour. Sci., Vol. X, 1875, p. 34. 



2 For a general discussion of this matter, see Williams' Geological Biology. 



