PETROLOGY, 289 



It has also been noticed that the existence of these four indepen- 

 dent-series constitutes a source of great confusion wherever the age 

 of the rock is not unmistakably established by the most distinct 

 field evidence. For instance, three at least of these series, the 

 groups 1, 2 and 4, consist of rocks which vary considerably in chemi- 

 cal composition, and in the present state of our knowledge of the 

 geology of this region, it is often impossible to tell to which group 

 certain intrusive rocks may belong. The volcanic formations of 4b 

 have not been sufficiently denuded to exhibit any of their intrusive 

 portions, but a basic intrusion might belong to any of the other sub- 

 divisions, while a more acid rock might belong indiscriminately to 

 1, 2, or perhaps 40. Even in the case of a lava, as in the instance 

 seen at Roba*t (pages 30 and 88), it becomes difficult to tell 

 whether it belongs to an exceptionally unaltered outcrop of group 1, 

 or an exceptionally disturbed outcrop of 4a, 



In many cases, however, there can be no doubt as to the correct 

 attribution of the rocks ; and, although their petrological characters 

 have been described in the works above mentioned, yet it is 

 interesting to be able to group them according to their relative 

 ages. 



The rocks which are classed together as group 1, that is the 

 volcanic rocks of upper cretaceous and lower eocene age, are 

 mostly ash-beds and tuffs which often consist of intermingled frag- 

 ments of widely different petrological characters. Almost every 

 variety of lava is represented in these fragments, from a quartz- 

 andesite to a highly basic basalt. A few lava- flows have also been 

 observed, whose structure is usually somewhat brecciated, as is very 

 commonly seen in rocks of this nature when they have been poured 

 out under water. 



The great intrusions and smaller dykes and bosses of group 2, 

 which have been several times noticed in the descriptive portion of 

 this Memoir, also vary in composition. Granites, syenites and 

 diorites, both with and without quartz, have been met with. They 

 often possess a truly granitic structure, or else, as at the edges of the 



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