342 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



ibly established. Nevertheless there are to be found among them 

 instances of rocks possessing a characteristic hitherto almost ex- 

 clusively ascribed to those of sedimentary origin. This is no other 

 than the arrangement of some of the constituents of these rocks 

 in a direction parallel to certain planes or lines. There exist 

 numerous instances of undoubtedly igneous rocks possessing par- 

 allel structure as marked as that of many sedimentary rocks. 

 Many trachytic porphyries possess this, especially those from the 

 Island of Ponza and Palmarola, from the foot of the Oyamel in 

 Mexico and from the mountain Pagus near Smyrna.* Hoffmann 

 also describes a trachyte from the Island of Pantellaria betwixt 

 Sicily and Africa which consists of a light greenish grey compact 

 fundamental mass with crystals of sanidine and another mineral, 

 which by their form, position and distribution occasion a marked 

 schistose structure. Trachytes of this nature have been observed 

 in the Island of Basiluzzo betwixt Stromboli and Lipari, and in the 

 Duchy of Nassau, f Slaty trachytes are also of frequent occur- 

 rence and have been observed by Leopold von Buch at Angostura 

 and near Perexil in Teneriffe, and in the Canary Islands at the 

 Caldera of Tiraxana and at Mogan on Grancanaria. Also by 

 Burat in Velay, especially at St. Pierre Eynac at the Pas-de-Compain 

 and in the Monts-Dores.J The slaty trachytes described by Burat are 

 classed by other geologists among the Phonolites, which latter also 

 furnish most remarkable instances of parallel structure among ig- 

 neous rocks. Phonolites as a class possess this slaty structure, which 

 is caused by the parallel position of the tubular looking crystals of 

 felspar contained in it and on this account the rock can often be split 

 up into slates and flags. This slaty structure stands also in connection 

 with the form in which these rocks have been deposited . In phonolitic 

 mountains it is generally observed that the flags and the layerlike 

 subdivisions of the rock corresponding to them are arranged 

 around the axis of the mountain in a bellshaped system of strata, 

 the inclination of the latter decreasing as the summit of the moun- 

 tain is approached. This would seem to indicate that the parallel 

 structure was occasioned by the flow of the phonolitic material 

 from the opening in the summit over and down the sides of the 

 mountain. This view is further supported by the fact that many 

 lavas possess a marked linear parallel structure, sometimes com- 



* Lehrbuch, I, 632. 

 t Lehrbuch, I, 634. 

 $ Lehrbuch, I, 635. 



