140 



the optical characters of different portions, it is generally found that the inner 

 zones are more nearly allied to anorthite than those which form the 

 exterior. 



Another feature of considerable interest is this. The inner zones are 

 frequently not bounded by sharp angles, whereas the external and therefore 

 last-formed zones often show a gradual approach to definite crystalline form. 

 This seems to show that the conditions under which the development of the 

 porphyritic crystals commenced were not favourable to the production of 

 definite form, but that they became more and more favourable as the growth 

 progressed. Now, as already stated, there is reason to believe that the 

 porphyritic crystals of volcanic rocks have been brought up from below, and 

 if so this implies that the initial stages of crystalline growth took place 

 under conditions more or less similar to those under which plutonic rocks were 

 produced. The absence of sharp angles in the nuclei of zonal felspars corresponds 

 with this view ; for, as will be shown later on, the felspars of the gabbros 

 often occur as irregular grains and coarse granular aggregates. Consolidation 

 under great pressure appears therefore in many cases to be unfavourable to 

 the development of crystalline form. 



The porphyritic felspars have often been broken and deformed by 

 mechanical forces connected with the movement of the magma in which 

 they were developed ; and in some cases the magma appears to have exercised 

 a corrosive effect upon them. 



In addition to the large porphyritic crystals one usually observes a 

 number of small columnar crystals which give lath-shaped sections. In a 

 large number of basic rocks these occur to the exclusion of the former. They 

 are usually so small that nothing definite can be made out with reference to 

 them except by the use of the microscope. They are frequently referred to 

 in descriptions of microscopic sections as lath-shaped felspars ; but of course 

 this mode of expression is not strictly accurate, as the term lath-shaped has 

 reference to the appearance of the most striking sections and not to the form 

 of the felspar. The long axis of these small columnar crystals lies parallel 

 with the edge P/M, and the terminal faces are often ill-developed. Twinning 

 on the albite plan is usually, though not always present. Binary 

 twins are not uncommon, and these have often been taken to indicate 

 the presence of orthoclase. Inclusions and zonal banding are usually absent 

 from the small columnar crystals. 



In the more coarsely crystalline rocks of the basic group the felspars 

 are frequently present in the form of irregular grains without any marked 

 approach to definite crystalline form. This is especially the case in those 

 rocks which exhibit the granitic texture in the greatest perfection; as, for 

 example, certain gabbros and norites. Sometimes, however, in these rocks 

 large portions of the felspar give definite extinction and include numerous 

 crystals or crystalline grains of the other constituents (see Plate III., 

 Fig. 2.) 



Liquid inclusions with moveable bubbles are frequently present in great 

 numbers in the felspars of the plutonic rocks of the basic group. They hare 



