THE PRIMORDIAL, OK CAMBRIAN AGE. 49 



The beds of the Primordial age exist in England, in 

 Bohemia, in Sweden and Norway, and also in North 

 America. They appear to have been deposited along 

 the shores of the old Laurentian continent^ and 

 probably some of them indicate very deep water. 

 The Primordial rocks are in many parts of the world 

 altered and hardened. They have often assumed a 

 slaty structure, and their bedding, and the fossils 

 which they contain, are both affected by this. The 

 usual view entertained as to what is called slaty struc- 

 ture is, that it depends on pressure, acting on more or 

 less compressible material in some direction usually 

 different from that of the bedding. Such pressure has 

 the effect of arranging all the flat particles as scales 

 of mica, etc. in planes parallel to the compressing 

 surface. Hence, if much material of this kind is 

 ^resent in the sediment, the whole rock assumes a 

 fissile character causing it to split readily into thin 

 plates. That such yielding to pressure has actually 

 taken place is seen very distinctly in microscopic 

 sections of some slaty rocks, which often show not 

 only a laminated structure, but an actual crumpling 

 on a small scale, causing them to assume almost the 

 aspect jf woody fibre. Such rocks often remind a 

 casual observer of decaying trunks of trees, and 

 sections of them under the microscope show the most 

 minute and delicate crumpling. It is also proved by 

 the condition of the fossils the beds contain. These 

 are often distorted, so that some of them are length- 

 ened and others shortened, and if specimens w?,re 



