Al^NIVERSAEY ADDEESS OF THE PRESIDENT. 49 



when the basis of fclie rock is no longer glassy. In quartz felsites, 

 like some of the Cornish el vans, we see larger enclosures of a 

 similar form ; but, instead of being glass, they are devitrified and 

 composed of fine-grained crystalline material, like the basis of the 

 rock. No such stony enclosures occur in very typical granites ; 

 but we do meet with a few in those somewhat approaching felsites 

 in general character. In the quartz of granites proper we see 

 enclosed crystals of other minerals; and these may be very 

 numerous and sufficiently characteristic of particular districts. 

 Thus, for example, small prisms of schorl so abound in the quartz 

 of Cornish granites, and fine needles of rutile in those of Scotland, 

 that sands derived from them would certainly show very charac- 

 teristic differences. I have been unable to detect any thing that 

 would serve to distinguish the quartz of thick-foHated schists from 

 that of true granite. In the case of thin-foliated schists it does, 

 indeed, often enclose plates of mica, chiefly lying in the plane of 

 foliation ; and if broken up, very characteristic fragments would 

 be formed, which might be called grains of mica-schist. 



The quartz of veins sometimes differs so little from that of 

 granite that I do not see how they could be distinguished. That 

 from large massive veins could not fall to pieces to form sand, but 

 would yield pebbles and small chips or granules ; so that we can 

 scarcely look upon it as an important source of sand of medium 

 quality, Vv'hich is what I propose to consider more particularly. 

 Some of that derived from thick-foliated schists, and especially 

 from gneiss, would not differ matei'ially from that derived from 

 certain varieties of granite; and it would also be impossible 

 to say whether some had been derived from an abnormal granite, 

 or from the mixed products of a granite and a felsite. Though 

 we may thus expect to meet with many grains of quartz-sand 

 presenting no distinctive peculiarities, yet some grains may still 

 teach us their whole history, and throw much light on the true 

 nature of the associated material. 



Sand derived from other Minerals. 



The felspars, augite, hornblende, schorl, and mica met with in 

 different classes of rocks do not differ sufficiently in external form 

 or internal structure to enable us to form any very satisfactory 

 opinion as to their origin, except in a few particular cases. Thus, 

 for example, the presence, or complete absence, of well-marked 

 portions of enclosed glass would indicate that any fragment under 

 examination was, or was not, derived from an igneous rock which 

 solidified from a state of true igneous fusion. This does not 

 necessarily prove that it was or was not a subaerial volcanic rock. 

 The presence of cavities of more or less perfectly spherical form 

 containing gas or vapour inside crystalline stony minerals, and the 

 absence of fluid- cavities, is, on the whole, the most satisfactory 

 evidence of their having been derived from a rock erupted under 

 little pressure, or as a true ash. On the contrary, the presence of 



VOL. xxxvi. / 



