332 



lavas of Snowdon and in the comparatively recent lavas of the Ponza 

 Isles so admirably described by SCROPE. The lamination is undoubtedly 

 the result of the deformation of a heterogeneous mass of plastic 

 material. Acid lavas when erupted at the surface are in a state of 

 imperfect fluidity, so that a perfect mixing of the portions which 

 differ in composition and physical properties is impossible. Thus, as the 

 whole mass takes the form of a flat sheet or elongated tongue, 

 according to the configuration of the surface near the point of 

 eruption, the individual portions of the mass take a corresponding 

 form and a banded or laminated structure is the result. If, as 

 frequently happens, the deforming influences are not uniform in their 

 operation crumplings and contortions are produced ; and, in the 

 extreme case, the more or less consolidated portions are broken up 

 and brecciated lavas are formed. 



Intimately associated with the different exposures of granite we 

 find veins and dykes of a rock largely composed of a substance 

 which cannot be resolved into distinct constituents by the naked 

 eye. This is the felsite of petrographers. Sometimes the rock is 

 wholly composed of felsite; more frequently we find porphyritic 

 crystals of quartz, felspar and one of the ferro-magnesian minerals 

 occurring as porphyritic constituents. The felspar may be monoclimc 

 or triclinic. The monoclinic felspar is usually in the condition of 

 orthoclase. When quartz occurs as one of the porphyritic constituents 

 the rock is termed quartz-felsite or quartz-porphyry ; when quartz is 

 absent as a porphyritic constituent and orthoclase present the rock 

 is termed orthoclase-felsite, orthofelsite, or orthophyre. The examina- 

 tion of thin slices under the microscope frequently enables one to 

 resolve the felsitic matter into an aggregate of crystalline constituents. 

 This, as we have already seen, has led to a subdivision of the por- 

 phyritic-felsites into micro-granites, granophyres and felsophyres. In 

 the micro-granites the felsite is composed of an aggregate of granitic 

 minerals arranged without any regularity; in the granophyres it is 

 mainly composed of quartz and felspar intergrown so as to form 

 micro-pegmatitic or pseudo-spherulitic aggregates ; in the felsophyres 

 it is variable in character and incapable of precise definition. 



When a mass of granite is traced towards its margin and from 

 thence into the apophyses which run out into the surrounding rocks 

 it is often seen to pass into micro-granite, granophyre, or felsophyre; 

 sometimes into a felsite without porphyritic constituents. Felsitic 

 dykes may sometimes be seen to cut both granite and the surrounding 

 rocks, and when this is the case the felsite is frequently similar to 

 a rock which occurs as contemporaneous veins in the granite. The 

 occurrence of glass as a constituent of the dykes and veins which 

 can be directly connected with granite masses is rare, but it has 

 been observed by LOSSEN (I) 



(1) Die Bode-gang im Hartz, Z.D.G.G. (1874), Vol. XXVI., p. 856. 



