ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. -349 



lie parallel with the walls of the vein. The trap of Kerrera de- 

 eribed by Macculloch is another instance of slaty texture in trap 

 veins. In this case the rock constituting the sides of the vein is 

 filled with scales of mica, which all lie parallel to the enclosing 

 walls. The same author also remaks concerning the hypersthenite 

 of Sky, that the crystals of hypersthene " are laminar and 

 " placed in a position parallel to each other, and as in gneiss to the 

 " plane of the bed m which they lie." Another peculiarity, 

 which shews the influence of igneous flow on the structure of a 

 rock is the following : The amygdaloidal varieties of melaphyr 

 sometimes possess an arrangement of their cavities corresponding to 

 that possessed by the gas bubbles of lavas. They are often elon- 

 gated, in which case their longest axes lie parallel to each other, and 

 we may suppose as in the case of lava, to the direction of the flow 

 of the igneous material. 



Granite and Syenite. — These formations include the oldest 

 ■eruptive rocks, the granites and gneiss-granites, which during the 

 primitive period seem to have broken through the comparatively 

 thin erust of the earth then existing. Later granitic eruptions 

 seem to have taken place with great frequency throughout the 

 Silurian, Devonian and Carboniferous periods, after which they 

 gradually disappear. Syenite does not seem to appear in the Primi- 

 tive G-neiss formation until long after the first general dislocation of 

 the same and the protrusion of the granite had taken place. The 

 principal syenitic eruptions seem to have occurred during or shortly 

 after the deposition of the Silurian and Devonian rocks, although 

 there are many instances of much younger syenites. The rocks 

 appear after their protrusion to have assumed all the forms of 

 occurrence, which we are accustomed to observe in plutonic and 

 even volcanic formations ; irregular masses, covers, (nappes), 

 layers and veins ; every form except the stream of the volcanic 

 rock. But it is to be remarked that instead of veins or dykes 

 being the most common form, as in the newer plutonic formations, 

 the irregular masses preponderate. These masses are not to be 

 confounded with the covers or cap rocks of the basalt and trachyte 

 formations. They are huge islands of granite as it were, possessing 

 generally an elliptical shape, and occurring in the midst of stratified ■ 

 roeks, which are sometimes vertical, and which often lean against 

 the granite as if it were the immediate cause of their inclined 

 position. One of the most important phenomena observable with 

 regard to granite in all its forms of occurrence is the extent to 



