LOCKS *25t 



kOgical periods (or metamorphic phases of the earth's crust) 

 other substances dissolved in vapors have issued from the in- 

 terior of the earth simultaneously with the eruption of granite, 

 basalt, greenstone porphyry, and serpentine. This seems a 

 fitting place again to draw attention to the fact that, accord- 

 ing to the admirable views of modern geognosy, the meta- 

 morphism of rocks is not a mere phenomenon of contact, limit- 

 ed to the effect produced by the apposition of two rocks, since 

 it comprehends all the generic phenomena that have accom- 

 panied the appearance of a particular erupted mass. Even 

 where there is no immediate contact, the proximity of such a 

 mass gives rise to modifications of solidification, cohesion, gran- 

 ulation, and crystallization. 



All eruptive rocks penetrate, as ramifying veins, either into 

 the sedimentary strata, or into other equally endogenous mass- 

 es ; but there is a special importance to be attached to the 

 difference manifested between Plutonic rocks* (granite, por- 

 phyry, and serpentine) and those termed volcanic in the strict 

 sense of the word (as trachyte, basalt, and lava). The rocks 

 produced by the activity of our present volcanoes appear as 

 band-like streams, but by the confluence of several of them 

 they may form an extended basin. Wherever it has been 

 possible to trace basaltic eruptions, they have generally been 

 found to terminate in slender threads. Examples of these 

 narrow openings may be found in three places in Germany : 

 in the " FJiast^r-kaute,'' at Marksuhl, eight miles from Ei- 

 senach ; in the blue " Kuppe,'' near Eschwege, on the banks 

 of the Werra ; and in the Druidical stone on the Hollert road 

 (Siegen), where the basalt has broken through the variegated 

 sandstone and gray wacke slate, and has spread itself into cup- 

 like fungoid enlargements, which are either grouped together 

 like rows of columns, or are sometimes stratified in thin 1am- 

 insB. The case is otherwise with granite, syenite, quartzose 

 porphyry, serpentine, and the whole series of unstratified com- 

 pact rocks, to which, from a predilection for a mythological 

 nomenclature, the term Plutonic has been applied. These, 

 with the exception of occasional veins, were probably not 

 erupted in a state of fusion, but merely in a softened condi- 

 tion ; not from narrow fissures, but from long and widely-ex. 

 tending gorges. They have been protruded, but have not 

 flowed forth, and are found, not in streams like lava, but in 

 extended masses. t Some groups of dolerite and trachyte in- 



* [Lyell, Principles of Geology, vol, iii., p. 353 and 359.] — Tr 



t The description here given of the relations of position under vi^hicb 



