OF VOLCANIC ROCKS. 59 



order of time in which trachyte and rhyolite have been ejected to the surface, is 

 corroborated by the fact that these rocks occupy generally a subordinate position in 

 regard to quantity, and have had, to a. great extent, their origin in volcanic action. 

 When treating about the latter, we will come back upon this subject. There remain 

 some peculiar features of volcanic rocks which cannot be satisfactorily explained at 

 the present time. We mention, among them, the fact that the three modes of texture 

 of rhyolitic rocks are often severally limited to certain localities ; the mode of forma- 

 tion of the laminated structure of rhyolitic and trachytic rocks ; the occurrence of the 

 compounds of hornblende and oligoclase in that threefold form to which we have 

 repeatedly referred ; the fact that basalt has been followed only to a very limited 

 extent by rocks bearing to it a similar relation, as trachyte and rhyolite do to andesite. 



As regards the long lapse of time intermediate between the Devonian and the 

 Tertiary periods, the mode of occurrence of eruptive rocks in the same shows in 

 nearly every respect a gradual transition from that which was peculiar to the granitic to 

 that which we just described as being characteristic of the volcanic era. This interme- 

 diate period may be designated as the porphyritic era, though this name appears to 

 apply more properly to its first part only. Quartziferous rocks were not so predom- 

 inant in it as in the granitic era, porphyrite and melaphyr having nearly equaled 

 quartzose porphyry, in point of quantity. Augitic porphyry was ejected in a 

 much larger proportion to the aggregate bulk of the porphyritic, than diabase to 

 that of the granitic rocks. Where it occurs, it was the last in the order of rocks 

 erupted, while quartzose porphyry was generally the first among them, though this 

 place is sometimes occupied by porphyrite. In reference to the general features of 

 their geographical distribution, porphyritic rocks occupy no less distinctly an inter- 

 mediate position, as may be seen by what we have said on this topic on another page. 



There are exceptions to the order of general development as here specified. 

 They regard chiefly the texture, and are almost exclusively to be found among the 

 rocks of the porphyritic era, though the recurrence, in propylite, of the properties of 

 ancient diorite, is a phenomenon of a no less exceptional nature. Leaving this rock 

 (propylite), or rather only some of its varieties, out of consideration, the volcanic rocks 

 have their peculiar characters, by which even the most basic rocks are to be recog- 

 nized when seen in large accumulation. In the rocks of the granitic era, if we 

 consider its end to be Within the Devonian period, the characteristic features of por- 

 phyritic and volcanic rocks are probably never to be observed. Among the excep- 

 tional occurrences within the porphyritic era, may first be noticed the fact that perfect 

 granitic texture is still occasionally encountered, as, for instance, near Predazzo and 

 on the Monzoni in southern Tyrol, where some subordinate masses of rock resembling 

 granite and syenite have been ejected in the Triassic age, together with the well-known 

 porphyritic rocks of that region. There are similar instances known from other places 

 on the European continent, but they are scattered, and the respective rocks always 

 quite limited in extent. The grandest exceptional instauce that is known up to this 

 time, is the recurrence in the Jurassic period of jierfect granitic texture in the erup- 

 tive rocks of the Sierra Nevada. But as regards their mineral composition, these rocks 

 belong to the family of syenitic granite, containing hornblende as a very characteris- 

 ed) 



