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special characteristic of the crystalline schists. In these rocks the principal 

 mineral constituents appear to have crystallised simultaneously, not succes- 

 sively. In a typical hornblende- schist, for example, composed of hornblende, 

 felspar and quartz, none of these minerals possess definite external form. 

 There is no indication of one of these minerals having been formed before 

 another. The author has shown that a hornblende-schist of the above type 

 may be produced by the metamorphism of a dolerite (diabase). The minerals 

 of the schist appear to be almost entirely of secondary origin and to be due 

 to the molecular re-arrangement of the original constituents under enormous 

 pressure, without any great elevation of temperature, or at any rate without 

 anything like a refusion of the original rock. If this be admitted, then it 

 follows that a holo-crystalline rock having a certain definite chemical 

 composition, may be produced either by the successive formation of crystals 

 during the cooling down of an igneous magma or by the simultaneous 

 formation of crystals in a solid mass at a lower temperature but under 

 enormous pressure. The minerals formed in the two cases will in general 

 not be the same. 



"We have now to consider other textural peculiarities of igneous rocks. 

 Holocrystalline non-porphyritic rocks present several varieties of texture 

 depending on the mutual arrangement of the constituent minerals. The 

 term granitic is used to express that texture which is the special characteristic 

 of granites. When the grain of the rock is so fine that the texture can only 

 be made out by the use of the microscope it is said to be micro-granitic. 



The term pegmatitic is used to express the texture which is well 

 illustrated by graphic granite. In this case the rock, or portion of the rock, 

 to which the term is applied is composed of two minerals which occur in more 

 or less isolated patches, but in such a way that different patches of one and 

 the same mineral possess the same optic orientation over a considerable 

 area, and behave therefore as if they belonged to one and the same crystalline 

 individual. Thus, when a patch of considerable size is examined under 

 crossed nicols, it is observed that in one position of the stage all the separate 

 portions of one mineral are extinguished, and in another position all the 

 separate portions of the other mineral. Quartz and felspar are the substances 

 which show the texture most frequently. When it can only be made out 

 by the use of the microscope it is said to be miero-pegmatitic. It sometimes 

 happens that the two minerals have a kind of radial arrangement, one of 

 them forming, as it were, a kind of nucleus. To express this texture the 

 term " centric " is used. 



Although the "centric" and "pegmatitic" textures are especially 

 characteristic of quartz and felspar they are exhibited also by other minerals, 

 as, for instance, garnet and felspar, omphacite and felspar, augite and 

 felspar, orthoclase and plagioclase. (1) Professor ROSENBUSCH uses the 

 term granophyric to express these textures when they are due to quartz and 

 felspar. 



Another very important variety of texture is that which M. LVY 



(1) F. BECKK. T.M.M. Neue Folge, Band IV., Plate III., Figs. 11, 12, 15, 17. 



