and their Contact Zones. 37 



It is that in many of the prisms there is a nucleus of 

 what I believe to be crypto-crystalline ground mass, 

 round which the hornblende individual has crystallised. 

 This is, therefore, quite different to those inclusions of 

 small spherules of ground mass which so often are seen 

 in the massive cleavable varieties. 



The principal inclusions in these two varieties of amphi- 

 bole are prisms of plagioclase and crystals, and masses of 

 magnetite and pyrite, and more rarely flakes of magnesia 

 mica. The included felspars are usually of very abnormal 

 structure, the twinning taking place in unusual modes. 

 The crystals are usually imperfect, the edges rounded, 

 or the sides unequal in dimensions. There may be only 

 one, or, again, a considerable number of striations, and 

 these striations are sometimes not continuous throughout 

 the slice, or may be confined to one-half, or be of different 

 directions in each half, or finally be represented by imperfect 

 laminae or flakes only. Many of these felspars have under- 

 gone serpentinous (?) alterations, together with or indepen- 

 dently of the hornblende. There are other inclusions 

 which I am unable to identify with anything else than 

 portions of crypto-crystalline ground mass, which has been 

 included. As I have said, these latter are often present 

 as cores of hornblende prisms, and then are found usually 

 to conform to the rhombic outline of the cleavage. 



The most usual alterations to which the hornblende has 

 been subjected is to some form of chlorite. The first 

 appearances of change are that the edge of the hornblende 

 becomes ragged and fibrous. The fibres are dichroic, in 

 shades of pale-yellow and bright emerald-green. In further 

 instances the hornblende is found to be wholly changed, with 

 the exception of a small portion, so that by such various 

 instances we are led to perceive the origin of similar 

 masses of chlorite where no traces whatever of hornblende 

 remain. In order to determine whether this chlorite is 

 fibrous or scaly, I have carefully traced round those examples 

 which occurred in the margins, and, therefore, in the thinnest 

 part of the slices, and I have there observed that the 

 chlorite fibres are approximately paralleled, are not of equal 

 length, and are, therefore, overlapping. In places the obser- 

 vations suggest that these fibrous appearances are due to 

 the sections being across plates or scales having a fibrous 

 structure ; and the very thinnest portions, examined with a 

 high power, appeared, indeed, to consist of minute pale-green 



