Xll. 



blow given in its vicinity. The glass will break, with the 

 formation of two sets of parallel fractures, which intersect 

 each other at a constant angle in a given piece (figs. vii. 

 and viii. ). 



Nomenclature.— In the study of the geological formations 

 of any region, it becomes necessary that a classification of 

 the various beds should be made, and that a proper nomen- 

 clature should exist, so that each division and subdivision 

 mav be properly designated. Professor H. S. Williams has 

 discussed the various systems of nomenclature which have 

 been used for stratified rocks, and for a full account the 

 reader is referred to his book.* A brief synopsis, and defini- 

 tions of the various terms employed, are given here. 



In the first place it must be remembered that we are 

 dealing both with the rocks, and with the time occupied in 

 their deposition. Consequently, a dual nomenclature and 

 classification is necessary, and two kinds of scales must be 

 adopted, namely: the "formation scale" and the "time 

 scale."!" The formation scale of classification takes account 

 of the rock formations only, while the time scale is only 

 concerned with geologic time and its subdivision. The time 

 scale will be considered first. 



The whole of geologic time is divided as follows : 



Cenozoic Time — Time of "modern life." 

 M esozoic Time — Time of "mediaeval life." 

 Pal.eozoic Time — Time of "ancient life." 

 Proterozoic Time — Time of "first life." 

 Azoic Time — Time of "no life." 



Each of these great "Time" divisions is characterized by 

 the progress of life during its continuance, such progress 

 being indicated by the names. 



•Geological Biology, 1x95. 



■ n S. Williams, Dual Nomenclature in Geological Classification. Journal of 

 logy, Vol. II., pp. 145-160. 



