of the Buchan District. 35 



of the rock structure. I found several places where there 

 were many fragments of other kinds of felsites embedded. 

 In one small area of about twelve feet in diameter I 

 observed embedded angular fragments of red quartz felsite,fine 

 grained grey felsite (felstone), compact grey felsite, and dark 

 coloured, almost black, felsite. The fragments were some 

 larger and some smaller than about an inch cube. I have 

 made the following observations on a thin slice of this 

 rock : — 



It has a large proportion of ground-mass, in which is so 

 large an amount of yellowish basis, that the slice remains 

 very much obscured throughout when rotated between 

 crossed nicols. It shows fluidal structure strongly. There 

 are many minute granules of quartz in the ground-mass 

 together with microscopic felspar prisms, which latter are 

 almost invariably arranged with their longer axes parallel 

 to the course of the flow. There is also a little iron ore 

 (magnetite ?) in grains, and finally many dark brown, to 

 almost black, long and somewhat ragged-edged microliths, 

 which are otherwise undefinable. In this ground-mass 

 are : — 



(a) Quartz crystals, with either crystalline outlines or as 

 fractured pieces. Sinuses of ground-mass penetrate them in 

 the usual manner, or the ground-mass separates fractured 

 parts. Fluid cavities are small and infrequent. The only in- 

 clusions I observed were portions of ground-mass and minute 

 portion of slaggy-looking material. 



(b) Felspars, some of which are orthoclase and some plagio- 

 clase. All have very imperfect outlines, more especially the 

 former, as if partially refused after crystallisation. The 

 triclinic felspars have all, so far as I could obtain measure- 

 ments, low angles formed by the plane of vibration, namely, 

 between 2° and 15°, and are, therefore, probably either 

 albite or oligoclase. The felspars contain many minute green 

 flakes, which may be a chloritic mineral. 



(c) A few minerals having a long prismatic habit and a 

 rhombic cross section. They are much altered to viridite 

 and to ores of iron (magnetite ?). I was unable to observe 

 any dichroic effects in the small portions having a pale green 

 colour still remaining clear and translucent. The rhombic 

 cross sections have the angles of amphibole, to which group 

 I refer this mineral. 



Finally, there are a few crystals of apatite, of which the 

 greater number are included in the felspars. 



