AND THEIR DESCENDANTS 



which are also well developed in that country. To these 

 " bottom " rocks Logan gave the name of Laurentian. 

 For some time afterwards the same name was also applied 

 to the somewhat similar rocks which were found to under- 

 lie the Cambrian formation in Britain, but it was felt 

 safer to give the English rocks a more general name. 

 They are therefore now usually called Pre-Cambrian, 

 which simply means older than the Cambrian strata, or 

 Archaean. 



In Canada the total thickness of the Laurentian, Pre- 

 Cambrian, or Archaean rocks is now estimated at 50,000 

 feet. In Britain it is nothing like so great as this (though 

 still considerable); but the thickness of these extremely 

 old and altered rocks is a very difficult matter to deter- 

 mine, for all signs of the original stratification in them 

 have often been destroyed, and the rocks have been 

 so bent and folded that it is possible the same beds 

 may have been measured more than once in the same 

 section. 



It will be understood from some of the foregoing 

 sentences that the task of dating or classifying these early 

 rocks is one which is far from simple, and which has given 

 rise to many different opinions. We may here give 

 another example. " During the years between 1831 and 

 while Sedgwick was occupied in studying the rocks of 

 North Wales," writes Mr. W. Jerome Harrison, " another 



good example of a foliated rock. It is composed of the three 

 minerals, quartz, felspar, and mica, arranged in this foliated manner. 

 Mica schist, talc schist, and other rocks have a similar structure, 

 and are sometimes briefly called " schists." 



201 



