THE CAMBRIAN PERIOD 491 



of the present continent into land, and there is evidence that these 

 lands, like those of America, were subjected to protracted erosion 

 before the deposition of the Cambrian system, for it is generally 

 unconformable on older strata. 



The Cambrian formations of Europe, like those of America, are 

 mainly clastic. A considerable portion of the material involved 

 in them is coarse, and the strata are often ripple-marked, and 

 affected by cross-bedding and by sun-cracks, features which 

 show that a large part of the Cambrian sediments were laid down 

 in shallow water, and some of them where they were not contin- 

 uously covered by water. 



In Wales (Cambria), the country from which the system 

 received its name, the system has a thickness of 12,000 feet or more. 

 This great thickness is equalled or exceeded in Brittany. In 

 Scandinavia, on the other hand, where Lower, Middle, and Upper 

 Cambrian all are present, the aggregate thickness is sometimes no 

 more than 400 feet. In western Russia also it is thin. These 

 differences probably mean that sediments were being deposited 

 in some places many times as rapidly as in others. They probably 

 also mean that the sediments were sometimes spread over flats 

 under shallow water and sometimes on shelving bottoms where 

 the layers were inclined. The Middle Cambrian is much more 

 wide-spread than the Lower or Upper, showing that changes in the 

 relation of sea and land were in progress during the Cambriar 

 period, shifting the areas of erosion and sedimentation. 



The Cambrian strata of western Europe have been much folded 

 since their deposition. In central and eastern Europe, on the 

 other hand, they are essentially horizontal. Beds of clay which 

 are still plastic, and beds of sand which are still uncemented, are 

 here found in the system. 



No geographic change of great importance seems to have marked 

 the close of the Cambrian. In this respect, as in some others, the 

 Cambrian histories of Europe and North America correspond. 



Other countries. Cambrian rocks occur in various parts of 

 Siberia, China, India, Australia, and Tasmania, and in the north- 

 western part of Argentina, but their distribution outside of North 

 America and Europe is but poorly known. 



