170 THE DWYKA TILLITE 



ancient boulder-clays (tills) and moraines of Northern 

 Europe and America, countries that are no longer so 

 extensively covered with ice and snow as they used 

 to be. 



Had the striated boulders in the Dwyka boulder-beds 

 belonged to a less remote geological period than the 

 Palaeozoic era, no doubt would have been cast upon the 

 glacial origin of their peculiarities and of the striated 

 surfaces upon which the boulder-beds frequently rest, 

 but geologists were reluctant to admit that glacial 

 conditions could have prevailed so long ago in countries 

 that now enjoy temperate and sub-tropical climates. 



With time and increase of knowledge the existence of 

 glacial conditions in the Palaeozoic era, and in what 

 are now temperate or tropical latitudes, has been gener- 

 ally admitted, and year by year the accumulation of 

 evidence bearing upon the origin of the Dwyka, and 

 similar boulder-beds in South America, Asia, and Austra- 

 lia show more and more forcibly that glacial action 

 is the only agency capable of explaining the formation 

 of these deposits. 



The use of the term " conglomerate " to describe the 

 boulder-beds is not satisfactory, for the boulders and 

 pebbles are rarely so abundant as to bring the rock 

 within the usual definition of a conglomerate. The 

 rock is too hard, as a rule, to be termed a boulder- 

 clay, while the terms moraine and ground moraine are 

 hardly justified until we know more about the conditions 

 under which the deposits were formed. To obviate 

 these difficulties and to avoid the rather cumbersome 

 phrase "hardened boulder-clay," Prof. Penck sug- 



