FORMATIONS AND PHYSICAL HISTORY 357 



there is a bowlder-bearing formation (the Gaisa beds) resting on a 

 glacial I'd surface of crystalline rock. The Gaisa beds have been 

 thought to belong to the oldest part of the Cambrian system, or to 

 anti-date it. (2) Recent exploration in China 1 has made known 

 a thick formation (170 feet) of bowlder-bearing rock of glacial 

 origin, containing many striated bowlders of diverse sorts of rock 

 ( I iu r . 321) on the Yangtse River, in latitude 30. This formation 

 lies at the base of the Paleozoic, beneath the beds that carry 



Fig. 322. A glaciated boulder from the Cambrian till of Petersburg, South 

 Australia. (Howchin.) 



Cambrian trilobites. Glacial formations of early Cambrian age 

 have been found in Australia, and perhaps in South Africa. 2 



The profound climatic significance of these glacial formations 

 is obvious. The testimony of Cambrian fossils, on the other hand, 

 implies nearly uniform climatic conditions throughout all regions 

 where fossils have been found, and the wide spread of the sea during 

 the- later part of the period would seem to point to oceanic, rather 

 than continental, climates at that time. 



Duration of Cambrian Period 



There is no reliable estimate of the duration of the Cambrian 

 period. The destruction and removal to the sea of such large 

 volumes of rock as are represented by the sediments of the system 



1 Willis, Researches in China, Vol. II. 



- David, Report of International ( leological Congress at Mexico, 1007; and 

 Ilowchin Ouar. Jour. Geol. Soc., Vol. LXIV, p. 234, 1908, and Jour, of Geol., Vol. 

 XX, pp. 103-8. 



