20 



greater amount of iron and the bases usually associated with it, in the glass 

 of the inclusions. The feature referred to may, as ZIRKEL points out, be readily 

 explained by the variations in composition, which must take place in the fluid 

 magma as crystals separate out of it. The general effect of this separation, as 

 we shall see later on, is to deprive the magma of the bases lime, iron and 

 magnesia, and consequently to leave it richer in silica and the alkalies. 



The glass of an inclusion very frequently contains crystallites, microlites 

 and crystals, similar to those which occur in the ground-mass of the rock. 

 These may sometimes have been enclosed along with the glass ; but, in the 

 majority of cases, they are unquestionably due to crystallising action set up 

 in the glass subsequent to its enclosure. Good illustrations of these are also 

 to be seen in the Arran pitchstones already referred to. 



Another feature usually, but not invariably, observed in glass 

 inclusions is the presence of one or more bubbles. These bubbles may either 

 represent a vacuum or the presence of a certain amount of gas. The common 

 form is spherical or elliptical, but sometimes the bubbles are seen to be 

 curiously distorted (Fig. 10), and when this is the case they give evidence of the 

 solid nature of the contents of the inclusions. When microlites are present they 

 are sometimes seen to project into the interior of the bubbles. In all cases 

 these bubbles appear, under the microscope, to be surrounded by a broad black 

 border, due to the marked difference in the refractive indices of the contents 

 of the bubble and the surrounding glass. Usually the bubble is within the 

 boundary of the enclosure, but sometimes it projects into the crystal substance. 



Dr. SORBY regards the bubbles as due to the unequal contraction of the 

 enclosed magma and the surrounding crystal as both cool down to the tem- 

 perature at which the observation is made ; but Professor ZIRKEL (1) holds that 

 this explanation is certainly not true in all cases. If it were true, there should 

 be a constant relation between the size of the bubble and that of the inclusion 

 in the crystals of the same rock or, at any rate, in the same crystal ; but this 

 is not the case. Sometimes there is no bubble, at other times the bubble 

 occupies the greater portion of the enclosed space ; and every possible gradation 

 between these extremes may be observed. He is inclined, therefore, to regard 

 the bubble as representing a portion of gas which has been disengaged from 

 the magma, and enclosed along with the glass. The presence of a bubble 

 which is half in the glass and half in the surrounding crystal certainly seems 

 to require some such explanation as this. It is, of course, quite possible that 

 the explanation of Dr. SORBY may apply in certain cases, and that of 

 PROFESSOR ZIRKEL in others. 



The forms of glass inclusions are liable to a considerable amount of 

 variation. Sometimes they are spherical, or elliptical; at other times they are 

 extremely irregular. One form which is not at all uncommon is that of a 

 negative crystal. In this case the actual form is of course dependent on that 

 of the containing crystal ; it is in fact an inverse reproduction of the latter. 

 Such inclusions may frequently be observed in the quartz of certain pitch- 

 stones and f elstones, as for instance those from Arran ; in the felspars and 

 augites of many rocks ; and in the lencites of Vesuvian lavas. 



(1) Mikroskopische Beschaffenheit, &c. 1873, p. 67. 



