ANNITEESAKT ADDEESS OF THE PEESIDENT. 65 



treme example of blown sand, differing in a remarkable manner 

 from the Millstone G-rit. The grains seldom show any sharp 

 angles, and are usually so much worn that the original external 

 surface must have been almost wholly removed. A few are worn 

 down to nearly perfect spheres. Some are flat, as though derived 

 from schists ; but by far the greater part are as if derived from 

 granite and felsite, or from some rock of intermediate character. 

 Fluid- cavities are not common ; but when they occur they are some- 

 what large for granite. A few enclosed particles of felsite paste 

 may be seen, and also, in some cases, flakes of mica or crystals of 

 schorl. Occasionally we see grains of jasper. On the whole it 

 appears as though this sand had been derived from rocks of varying 

 type. 



Modern Sand of the Thames. — ■Flint-sand plays an important 

 part in the modern deposits of the Thames. By counting a large 

 number of grains seen under the microscope, I found that between 

 Eichmond and Brentford the proportion of flint to the entire 

 amount of siliceous sand is about 4| per cent., but decreases as we 

 pass down the river, until near Grravesend it is reduced to less 

 than 3 per cent. I have no doubt that a large part of this is 

 derived from the flint-gravel used to make or repair the roads, 

 since the drift from them contains 10 or 20 per cent., and occa- 

 sionally more. In the old deposits of the Thames valley, like the 

 sand of Hyde Park or the Thanet Sand, the amount of flint is only 

 1 per cent. ; and thus in such a district we might form some idea 

 as to the relative age of different more or less modern deposits. I 

 may also here allude to the occasional large amount of coke dust 

 and rounded or angular grains of coal in very modern sand beds 

 or mud banks formed in the neighbourhood of a large popu- 

 lation. 



Terrace-sand in the valley of the Tay at Dunheld. — This is a good 

 example of sand derived from schists. The most striking feature 

 is that the fragments of quartz are, on the whole, much flattened, 

 and contain very few fluid-cavities. These are associated with 

 many flakes of green and colourless mica, and with fragments of 

 hornblende, as well as a few of garnet and other minerals. Amongst 

 the coarser grains are many of true mica-schist not broken up into 

 its separate constituents. 



Trias Sand at Paignton. — This contains many rounded frag- 

 ments of what appears to have been a hard slate rock, distinguished 

 from grains of partially decomposed felspar by the presence of the 

 usual minute hair-like crystals of black oxide of iron lying in the 

 plane of cleavage. The associated quartz grains are a little, but 

 not much, worn, and indicate that they were derived from both 

 granites and schists. 



These few examples must be looked upon merely as illustrations 

 of what may be learned by studying the grains in detail; and 

 further and more striking examples will be described when treat- 

 ing of slate-rocks and schists. 



VOL. xxxvi. q 



