PETROGRAPHY, 351 



magma. The various magmas solidified sometimes under- 

 ground as laccolites, sometimes as dykes, sometimes as super- 

 ficial flows, and induced contact-metamorphism of diverse 

 characters. Broegger could not determine any definite 

 sequence in the separation of the component minerals, but 

 was able to add many observations bearing upon this point. 



A later publication by the same author is entitled The 

 Eruptive Rocks of the Christiania District, and comprises two 

 volumes. The first, published in 1894, is devoted to the 

 rocks of the Grorudite - Tinguaite series. Broegger thinks 

 these take an intermediate position between deep-seated bosses 

 and eruptive flows, and represent members of a connected 

 series of protrusions from the same magma, which either 

 solidified underground in massive form or occupied crust- 

 fissures. 



The second volume, published in 1895, instituted a com- 

 parison between the succession of eruptive rocks in the 

 Christiania district and that in the eruptive district of Monzoni 

 and Predazzo in South Tyrol. On the basis of his own 

 observations in both districts, Broegger explains the famous 

 Triassic monzonite, granite, and hypersthenite as deep-seated 

 laccolitic expansions of the eruptive flows in the same neigh- 

 bourhood (melaphyre, augite, porphyrite, and plagioclase 

 porphyrite), and views them as a series of differentiations 

 from an originally uniform magma, analogous with the differ- 

 entiations presented in the Christiania district. 



From the foregoing pages it is apparent that Rosenbusch, 

 Broegger, Iddings, Williams, and others are inclined to 

 minimise the petrographical contrast between the so-called 

 plutonic and volcanic rocks, and to recognise in underground 

 and superficial occurrences of eruptive rock only different 

 facies of the same magma, consolidated under different con- 

 ditions. On the other hand, Zirkel (1893) strongly emphasised 

 the differences between the granitic, deep-seated rocks and the 

 porphyritic or glassy flows, and brings forward many objections 

 to the laws enunciated by Rosenbusch regarding the succes- 

 sive separation of minerals from fused masses. In general, 

 petrographers may be said to be still actively investigating the 

 ground-masses, in the study of which there are many problems 

 awaiting solution. 



Microscopic researches have fully elucidated the composition 

 and the origin of the sedimentary strata. There is no longer 



