144 MR J. D. FALCONER ON THE 



The Intersertal Material or Mesostasis. 



Throughout the dykes and the marginal portions of the sills, the intersertal material, 

 which, though always crystalline, may be spoken of generally as mesostasis, is found in 

 comparatively small quantity. Locally it is aggregated with felspar into knots which 

 are readily recognised by their imparting a red or grey spotted appearance to the rock. 

 The greatest development of mesostasis, however, is found in the central portions of the 

 larger sills, and there perhaps most abundantly in the pale-coloured felspathic modifi- 

 cations. There also its nature can be most readily investigated. Under the microscope 

 the angular spaces between the columnar felspars and augites are partially or wholly 

 filled with a micropegmatitic growth, in which the proportions of quartz and felspar 

 may apparently vary considerably within a few millimetres. (See PL II. figs. 2, 4, 5.) 

 The amount of quartz increases, as a rule, towards the centres of the intersertal spaces, 

 and angular or irregular portions of the same mineral are frequently found embedded in 

 the linear micropegmatite. These may be either earlier crystallisations or later corrosion 

 effects. (See PI. II. fig. 5.) As a rule, a band of linear micropegmatite lines, as it 

 were, the walls of the intersertal cavity, especially where these are formed of plagioclase, 

 with which the later felspar is frequently in optical continuity. Where an augite forms 

 a portion of the wall it is only where there has been an abundant supply of material 

 that it also is provided with a micropegmatitic fringe. Usually there is a gap in the 

 continuity of the lining opposite to the augite. The interiors of the intersertal spaces 

 are very generally filled with an allotriomorphic micropegmatitic growth from various 

 centres, or more rarely by a mass of cryptocrystalline material, or by a trachytic 

 aggregate of small felspar crystals with a little interstitial quartz. Occasionally, also, a 

 coarse-grained micropegmatite or a granular aggregate of quartz and felspar crystals, or 

 a mass of felspar or of quartz alone may occupy the centre. Small idiomorphic felspar 

 crystals may sometimes be seen projecting from the walls into such a central mass of 

 quartz. Micromiarolitic cavities are also occasionally observed with the walls lined 

 with tiny idiomorphic crystals of quartz, and the interior loosely filled with secondary 

 chloritic material. Curiously enough, masses of quartz, sometimes with miarolitic 

 cavities, are frequently found, not in the centre of an intersertal space but marginally, 

 and in the neighbourhood of an augite crystal where such happens to form the wall, 

 and to be devoid of a pegmatitic fringe. In such a case the quartz usually moulds the 

 augite, and has apparently also corroded it to some slight extent, irregular portions of 

 quartz being sometimes found embedded in the hornblendic margins of the augite. 



It is evident that the fringes of linear micropegmatite have crystallised in regular 

 series after the compound labradorite-oligoclase crystals, but it is equally evident that 

 they were not always the last product of crystallisation. Masses of felspar or of quartz 

 or a granular or cryptocrystalline aggregate of both may apparently succeed the micro- 

 pegmatite in period. The fringes themselves frequently assume very beautiful forms, 

 resembling in every respect the diagrams of "micropegmatite a etoilement" and 



