[4] 



surfaces still deeper forming sands, clays, and gravels, and forcing 

 these down to the sea-shore ; and icebergs, many miles in circumference, 

 are carried by currents along coasts and against cliffs like huge ploughs, 

 completely altering the face of the rocks beneath. This wear and tear 

 results in the formation of immense quantities of detritus, which is 

 deposited in layers at the bottom of seas and rivers, and consolidated 

 by pressure, being frequently assisted by lime, iron, or silica as a 

 cement. The coarser-textured rock has been laid down in rapidly- 

 moving, shallow water ; and the finer-textured in still, deep water. 

 Thus, through many long ages probably millions of years the surface 

 of the earth underwent continual change from the constant deposition 

 of stratified rock, each layer of which completely buried beneath it the 

 various life forms of the previous period, which circumstance enables 

 us to ascribe to the various members of the animal and vegetable king- 

 doms particular geological periods ; for fossilised remains of animals 

 and vegetables have been unearthed in the different layers of the 

 stratified rocks, conclusively proving their existence on the earth at 

 those periods. 



In the Plutonic or unstratified rock-formation period there was, of 

 course, no life upon the earth, the conditions necessary for such develop- 

 ment not being present ; but in the very earliest of the stratified forma- 

 tions we find evidence of the dawn of marine life, both vegetable and 

 animal. Geologists have divided the stratified rock into three chief 

 divisions, the Palaeozoic (ancient life), or Primary; the Mesozoic (middle 

 life), or Secondary ; and the Kainozoic (latest life), or Tertiary. Each 

 of these, again, has been subdivided into smaller sections, according to 

 the particular kind of deposit met with, the particular places where the 

 best examples are to be found, or the particular life-forms existing. The 

 Primary, the depth of which is unknown, is subdivided into seven 

 periods viz. : 



Laurentian, consisting of highly metamorphosed (that is, changed 

 in appearance from the original stratified rock character, owing to its 

 proximity to the molten Plutonic rock) limestone, containing fossil 

 remains of the Foraminifera, some of the first living organisms. 



Huronian, consisting of less highly metamorphosed sandstone, lime- 

 stone, etc., and containing fossil remains of lowly-organised molluscs 

 (soft-bodied organisms). 



Cambrian, consisting of slates, sandstones, and conglomerates, and 



