35 



such circumstances. Tn these rocks the porphyritic texture is comparatively rare 

 and the separation of the primary minerals into the two classes above referred 

 to becomes in many cases impossible. 



Minerals of the fourth group are termed secondary. They owe their exist- 

 ence to a variety of causes many of which are very imperfectly understood. 

 In the first place we have to notice the fact that it by no means follows that 

 all the minerals which are formed during the process of consolidation result 

 from the direct separation of crystals from the molten magma. In certain 

 cases minerals are produced by the action of the magma on crystals previously 

 formed or on the minerals and fragments which are foreign to the rock mass. 

 The hornblende crystals of certain andesites and porphyrites are often sur- 

 rounded by a dark border which may be sometimes resolved, by the use of a 

 high power, into an aggregate of magnetite and augite granules. In this case 

 the two latter minerals must be regarded as due to secondary processes operat- 

 ing before the final consolidation of the rock. As an illustration of the de- 

 velopment of minerals in consequence of the action of the magma on foreign 

 substances, the very interesting case of the production of bipyramidal quartz- 

 crystals at the zone of contact of a basaltic magma with sandstone and other 

 inclusions, described by J. LEHMANN, (1) may be instanced. In this case 

 there can be little doubt that the basaltic magma has taken up silica from 

 the inclusion so that, as cooling progressed, the solution at the zone of contact 

 became supersaturated and a crystallisation of quartz took place. 



The secondary minerals, however, owe their origin, as a rule, to agencies 

 which have operated subsequently to the consolidation of the rock. There are 

 for instance the surface agencies which depend largely on the passage of water 

 through the mass of the rock and give rise to the formation of such minerals 

 as chlorite, epidote, serpentine, carbonates and zeolites ; there are deep-seated 

 agencies which depend on the action of water, acid gases and other substances 

 at a high temperature and under great pressure ; and lastly, there are the 

 mechanical agencies which produce the phenomena of regional metamorphism. 

 By the operation of one or more of these agencies igneous rocks may be more 

 or less modified, and in some districts every gradation may be found from a 

 rock possessing all its original characters to one in which there has been a 

 complete molecular re-adjustment and a destruction of all the original textural 

 characteristics. A massive dolerite (diabase), for instance, may be con- 

 verted into a chloritic-'-J or hornblendic-schist. (8) 



(1) Die pyrogeneri Quarze in der Laven des Niederrheins. Verb. d. naturh. Vereins. Bonn 

 1877. 



(2) Lessen. Studien an metamorphiscben Erupriv-. und Sediinentgesteinen. Jahr. d. k. 

 1'ieiiss. Geol. Landesanstalt. Berlin, 1884. p. 628. 



,3) Teall. Metamorphosis of dolerite into hornblende-schist. QJ.G.S. Vol. XLI. p. 133. 



