21 



The arrangement of the inclusions in the crystal also varies in different 

 cases. Sometimes they are crowded together in the centre of a crystal, 

 sometimes they form a band round the periphery, and very frequently they are 

 arranged in a zonal manner ; the zones corresponding in form with that of 

 the crystal itself. Five or six distinct zones may sometimes he detected, as for 

 instance, in the felspars of the andesites. It is impossible to avoid the conclusion 

 that the zonal arrangement indicates variations in the conditions of crystal- 

 growth; and it is probable that, as in Dr. SORBY'S experiments, it is the 

 rate of growth that determines the number of inclusions. The more rapid the 

 growth the greater the number ; the slower the growth the smaller the 

 number. With regard to the precise way in which inclusions arise we are, to 

 a very great extent, in the dark. Crystals grow by accretion ; that is by the 

 addition of matter to their external surfaces. If this addition be regular and 

 constant during the period of growth then a homogeneous crystal will be the 

 result ; but if, for some unknown reason, the addition of material be interrupted 

 at certain points, then indentations will be formed on the growing surfaces and 

 these indentations may become inclusions by the growing together of their 

 walls as the crystal increases in size. Dr. SORBY'S observations on growing 

 salt crystals led him to the conclusion that this was the way in which 

 inclusions originate. It is well to note however that they may possibly, in 

 certain cases, be formed in another way. The quartz crystals of rhyolites, 

 felsites, and pitchstones are regarded as having been formed from an igneous 

 magma, and yet they frequently present appearances which imply that the 

 magma has exerted a solvent action on the crystal. Thus the angles are usually 

 rounded off, and the groundmass of the rock often penetrates into the crystal 

 in the form of miniature bays, gulfs and fiords. If both these views, which 

 at first sight appear to be mutually destructive, be correct, we are driven 

 to the conclusion that alterations in the physical conditions, and possibly 

 also in the chemical composition of a magma, may cause a crystal to be 

 developed at one time, and corroded at another ; and if repeated changes 

 in the conditions occur, so that a period of growth follows one of cor- 

 rision, it is quite possible that inclusions may be formed by the growth and 

 completion of corroded crystals. These considerations naturally lead us to 

 the subject of secondary glass-inclusions. 



For a long time it was supposed that glass inclusions must of necessity be 

 original. It has recently been shown by M. K. von CHRUSTSCHOFF (1) that this 

 is not the case. Secondary glass inclusions are found in the minerals of certain 

 rocks which have been acted upon by igneous magmas. They occur for instance 

 in fragments of sandstone, granite, gneiss and other rocks, which have been 

 caught up by molten material during its eruption or intrusion. They have 

 also been observed at the contact of eruptive masses and the surrounding 

 rocks (2) and have been produced artificially by subjecting fragments of 



(1) Tiber secundare Glaseinschliisse. T.M.M., Neue Folge, Vol. IV., p. 473, and Vol. 

 VII., p. 64. 



(2) At the contact of granite and melaphyre, Petrographische Studie am Granit von Predazzo. 

 A. Sigmund. J.G.R., vol. XXIX.. 1879, p. 311. See also CHRUSTSCHOFF, Uber den Granit des 

 Monte Mulatto, Predazzo. N,J., 1885, Band II., p. 66. 



