176 Proceedings of the Royal Physical Society, 



large boulders, as we have sufficiently seen, bear striae in no 

 wise differing from those of glaciated rock. Pieces of coal or 

 shale the size and shape of a taper finger nail, on the other 

 hand, bear lines as minute and magnifiable as the delicate 

 lines on the finger nail. These little boulders have not 

 apparently been brought into rude and direct contact with 

 the rock or with each other. Their striae are just such as 

 would be caused if, carried along in a wrapping of clay, they 

 had been scratched by the particles of the matrix, and dealt 

 with gently and with approximate equalisation of pressure 

 (see Fig. 8). 



Structure of the Matrix. — Microscopic Boulders, 



It need not be said that this is not restricted to boulders 

 the size of the finger nail. It is possible to pick out from 

 well-wrought boulder-clay a scale of lessening sizes leading 

 down to a point at which the glaciated character of the 

 particles is lost to the naked eye. " If the clay be washed 

 and sifted," says Professor James Geikie, " it will be found to 

 be composed of grains of all shapes, sizes, and weights, down 

 to the finest and most impalpable flour." ^ The grains, in 

 fact, exhibit all the varieties of form that are found among 

 larger boulders. Many of them are angular ; others are in 

 various degrees of roundedness ; others, again, are of char- 

 acteristic oval and lobate boulder-shapes — some with the 

 indistinct longitudinal etching of surface that covers many 

 stones in the boulder-clay either too rough to take striae or 

 not long enough under the process, and some striated dis- 

 tinctly.^ Many little boulders can be detected, measuring 

 about the 100th part of an inch in length, and bearing stride 

 about the swo^^^ ^^ ^^ i^ch in diameter. 



Fig. 7 represents a few of the best striated and most char- 

 acteristically shaped boulders obtained by washing the well- 

 kneaded boulder-clay on the Fillyside shore free from the 



1 Intercrossing of Erratics, Scottish Naturalist, vol. vi., p. 249. 



2 I am indebted to Professor James Geikie for first informing me that he 

 had detected glacial stria on felspar grains, running transverse to the planes 

 of mineral striation ; and on the first occasion on which I looked out for it 

 myself I had the help of Mr B. N. Peach. 



