350 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



already become classic; generally speaking, they go to show 

 that the mineralogical character of the stratified rocks as 

 affecting the conduction of heat and the relative pressures 

 between the bedded rocks and the intruded igneous rock- 

 material, influenced the subsequent processes of consolidation 

 in the latter, and determined the orientation of crystals and 

 the modifications of structure. 



In many active and extinct volcanoes, it would appear that 

 the character of the ejected rock-material gradually alters with 

 each successive eruption, so that the first and the last products 

 of eruption represent the extremes of a petrographical series. 

 In the Rocky mountains, and in the Sierra Nevada, Baron von 

 Richthofen (1868) recognised a definite sequence of propy- 

 lite, andesite, trachyte, rhyolite, and basalt, and his observa- 

 tions have since been confirmed by American geologists. The 

 more recent works of Professor Broegger on the eruptive dis- 

 trict of Southern Norway have extended the observations so 

 ably initiated by Baron von Richthofen. Professor Broegger 

 has given an admirable exposition of the eruptive rocks in 

 that district with respect to their mineralogical, structural, 

 and chemical constitution, their geological occurrence, their 

 eruptive sequence, the division and differentiation of the 

 original magma. 



In the year 1890, Professor Broegger contributed a paper 

 to the Zeitschrift fur Krystallographie und Mine>alogie, in 

 which he sub-divided the eruptive rocks in the neighbourhood 

 of Christiania into two chief series, an older and a younger, 

 the younger containing only basic intrusive rocks (diabases), 

 the older comprising very different acid and basic rocks, which 

 may be again sub-divided into five groups according to their 

 mineralogical and chemical composition. All the products of 

 this older group form a transitional series of rocks passing 

 petrographically into one another, and closely related chemi- 

 cally. They have clearly proceeded from an originally con- 

 tinuous molten mass which has been segmented, and has 

 undergone differentiation into several rock-types. The oldest 

 members of the genetic series are basic, the youngest strongly 

 acid. In the opinion of Broegger, the original magma was an 

 aquo-igneous solution of silicates, and rich in soda. Towards 

 the close of the Devonian epoch, the first fissure eruptions 

 took place, the magma being still fairly basic, and these were 

 succeeded from time to time by outbreaks of increasingly acid 



