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 voleanie 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 rhyolitie and trachytic rocks ; the occurrence of the 
compounds of hornblende and oligoclase in that threefold form to whieh 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 voleanic 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 voleanie 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- 
phyritie and voleanic 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 instance that is known up to this 
time, is the recurrence in the Jurassic period of perfect 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- 
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