THE CAUSE OF THE GLACIAL PERIOD. 491 



years. It is now well ascertained that there have been several 

 glacial periods but at irregular and widely separated intervals. 

 The irregularity of the intervals would indicate that the cause 

 cannot be the astronomical changes which Mr. Croll had 

 adduced, for they occur at regular intervals. The facts as 

 summarized by Professor Coleman are as follows:* 



(1) The Huronian rocks of Canada, which are the oldest 

 sedimentary strata in existence, contain extensive conglom- 

 erates in every area mapped in northern Canada through a 

 region 1,000 miles long from east to west and 750 miles broad. 

 Some of the bowlders in this conglomerate are tons in weight, 

 while striated pebbles are as characteristic of glacial deposits 

 as can be found in any other age. Sir Archibald Geikie had 

 also noted similar deposits of archaean age in Scotland. The 

 deposits in Canada occur over hundreds of thousands of square 

 miles. 



(2) In rocks of early Cambrian age extensive conglom- 

 erates such as would be formed from the petrification of 

 glacial till are found in widely scattered regions, more specially 

 of the southern hemisphere. From such deposits in China 

 Mr. Baily Willis has recently brought back beautifully 

 glaciated stones. But much larger areas in Australia and 

 South Africa are covered with "tillite" of Cambrian age. In 

 South Australia Mr. Howchin has traced these deposits over 

 an area extending 450 miles from north to south and 250 from 

 east to west, with a thickness of 1,500 feet. In South Africa 

 the Cambrian tillite has been traced by Mr. Rogers over an 

 area of 1,000 miles. In both regions the glaciated area lies 

 near the 30th degree of latitude, and the movement of bowlders 

 has apparently been from south towards the north. Similar, 

 but less clearly defined Cambrian tillite has been reported at 

 various places about Lake Superior. In Australia the ice 



*"Glacial Periods and Their Bearing on the Geological Theories," 

 'Bulletin of the Geological Society of America," vol. xix, pp.347-CCG. 



