407 



or possibly to causes of which we are at present totally ignorant. 

 The present relation of the different parts is probably in many cases 

 the result of movements taking place after the differentiation has 

 been effected. The fact that plutonic masses are often heterogeneous 

 is of great importance as bearing on the origin of those banded 

 crystalline rocks which are identical with plutonic rocks in com- 

 position/ 1 ) 



Plutonic magmas may, by the action of earth -stresses or by the 

 action of imprisoned vapours, be injected into the surrounding and 

 overlying rocks. Under these circumstances they will follow planes 

 of least resistance and take the form of dykes, sheets or laccolites. 

 The planes of least resistance may be faults, joints or planes of 

 stratification. Volcanic eruptions take place when a communication is 

 established between the plutonic masses 'and the surface of the earth. 

 The ultimate form taken by the ejected material will depend on the 

 character of the eruption. The molten matter which reaches the 

 surface is generally charged with crystals. These crystals have been 

 developed in the molten mass either before or during the act of 

 eruption. Very often their central portions have been formed under 

 plutonic conditions and their peripheral portions during the act of 

 ejection. They constitute the porphyritic constituents of the volcanic 

 rocks, and as illustrations we may mention the quartz and orthoclase 

 (sanidine) of the felsites and liparites, the plagioclase and pyroxene 

 of the andesites, and the plagioclase, olivine and pyroxene of certain 

 basalts and dolerites. Such crystals frequently occur in groups, and 

 when this is the case the internal relations of the crystals accord 

 with the view that they have been developed under plutonic con- 

 ditions the individuals of a group are often allotriomorphic internally 

 and idiornorphic externally. This feature is well illustrated by the 

 bytownite-aggregates in the Tynemouth dyke and the labradorite- 

 aggregates in the Cheviot andesite (see Fig. 1, Plate XXXVI.) 



The porphyritic crystals of the volcanic rocks frequently show 

 a zonal structure and the material of successive zones presents 

 different chemical and optical characters. This zonal structure is 

 best seen in the lime-soda felspars and in the augites. It is 

 due to isomorphous replacements and to change in the composition 

 of the magma during consolidation. As a consequence of this 

 zonal structure it follows that the analysis of one zoned individual 

 does not represent the composition of any portion of that individual 

 it is merely, as ROSENBUSCH says, a kind of bulk analysis. The 

 porphyritic crystals frequently give evidence of corrosion and some- 

 times also of crystalline growth having taken place after corrosion. 

 The corrosion may be due to physical or chemical changes during 

 consolidation. Mere relief of pressure may, as LAGORIO points 

 out, have the effect of corroding crystals ; for, by SOREY'S principle, 



(1) See TEALL, on the Origin of certain Banded Gneisses, G.M., Decade III., Vol. IV., p. 484. 



