ANNIVEKSAET ADDEESS OE THE PEESIDENT. 85 



Structure of /Schists. 



It thus appears that we may trace, step by step, the change from 

 stratified deposits of a more or less normal type to rocks in which 

 nearly the whole of the material has recrystallized in situ, to form 

 truly metamorphic schists ; but, at the same time, it does not neces- 

 sarily follow that all schists which have crystallized in situ were 

 originally composed of sand and clay. I therefore now propose to 

 examine what evidence still remains of their truly detrital origin, 

 and of their probable constitution before crystallization took place. 

 I scarcely need say that no class of microscopical rock-preparations 

 are more difficult to make than sections of some mica-schists, cut 

 at right angles to the foliation. By slow and careful grinding, and 

 by repeatedly covering the surface with Canada balsam and heating 

 until it became hard, so that it might penetrate between the plates 

 of mica and bind all together, I have, however, succeeded in pre- 

 paring faultless sections of the most intractable specimens, which 

 can be examined with as high magnifying powers as are necessary 

 for their efficient study. 



Presence of the Original Sand and Mud in Schists. 



In my paper on mica-schist, published many years ago in our 

 Journal (vol. xix. p. 401), I described what I looked upon as the 

 original grains of sand still visible in the altered rock. I have 

 lately reexamined my microscopical sections with improved appa- 

 ratus, and am now more convinced than ever that I was correct. 

 The best way of studying this question is to use a f inch object- 

 glass with an attached, parabolic, side reflector, in such a manner 

 that the object may be seen at will, either by surface-illumination 

 or by transmitted polarized light. When quartz from various 

 sources is thus examined, some appears quite transparent and 

 colourless by transmitted light, and almost black by surface-illu- 

 mination, whilst some is more or less opaque by transmitted light, 

 and milk-white by surface-illumination. A good deal is interme- 

 diate between these extremes, or shows a mottled mixture of both. 

 The whiteness is either of a granular character (when due to fluid- 

 cavities or large granules), or uniform and homogeneous (when the 

 foreign particles are too small to be separately visible). Some spe- 

 cimens are further characterized by the presence of many included 

 hair-like crystals. 



The quartz of many specimens of schist appears almost wholly 

 dark by surface-illumination ; but here and there, in certain speci- 

 mens from Dunkeld and Arroquhar, are milk-white grains, perfectly 

 unlike the surrounding quartz, having a well-defined, rounded, 

 waterworn, or more angular outline, just like the genuine and 

 undoubted grains of sand in more recent, unaltered, stratified rocks. 

 They further differ from the quartz of the schist itself in having a 

 simple optical structure, each being a part of a single crystal, like 

 most grains of sand ; whereas the small aggregations of quartz 

 crystallized in situ have a complex structure, and are made up of 



