BY H. 



JENSEN. 



495 



the latest period of volcanic action commenced late in the Creta- 

 ceous and ended in the Pliocene; but in Java, and the East 

 Indies generally, this period of vulcanicity commenced in the 

 early Tertiary and still continues in parts with unabated violence. 



We may conveniently say that in each geological province 

 there have been three great " pyrogenic " periods since the com- 

 mencement of the Palaeozoic era, the first falling exclusively in 

 the Palaeozoic (chiefly Devonian), the second commencing at the 

 close of the Palaeozoic and lasting well into the Mesozoic (chiefly 

 Permian), and the third commencing in the late Cretaceous and 

 lasting well into the Tertiary. These periods we may call, for 

 the sake of brevity, the Palaeopyrogenic, the Mesopyrogenic and 

 the Neopyrogenic, with the proviso understood that each does 

 not necessarily fall into the corresponding era of the geological 

 record. 



Before the commencement of life, in the Archaean or Azoic 

 periods, there was in every part of the earth intense vulcanicity. 



The following table seems to summarise these facts. 



Period. 



Azoic 



Precambrian 



Early Cambrian 



Late Cambrian 



Ordovician 



Silurian 



Early Devonian ...... 



Late Devonian 



Early Carboniferous 

 Late Carboniferous.. 



Permian ... 



Triassic 



' urassic 



Cretaceous 



Eocene 



Miocene 



Pliocene 



Pleistocene 



Recent. ,. „ : 



Dominant Tendency 



transition 



elevation 



stability 



submergence 



submergence 



stability and transition 



transition and uplift 



uplift 



submergence 



transition and uplift 



uplift 



stability 



submergence 



transition 



uplift 



uplift 



uplift 



stability 



Volcanic Phenomena 



very abundant 



very abundant 



infrequent 



rare 



rare 



rare 



more frequent 



very abundant 



frequent 



frequent 



very abundant 



very rare 



very rare 



very rare 



very abundant 



frequent 



less frequent 



infrequent 



rare 



If we consider the sequence of events in the East Australian 

 geological province, we arrive at the following set of conclusions: 



