7i6 PRINCIPLES OF STRATIGRAPHY 



they are derived is a thin-bedded rock or tends to split into thin 

 layers. Here the pebbles are shoved up the beach and dragged 

 back, rolling being seldom seen. Pebbles formed from angular 

 fragments, on the other hand, are rolled about on the beach. 

 Thus it appears that the original form of the material is of more 

 significance than the agent active in the rounding. 



14. Characteristics of Inclusions in Sand Grains. In 

 tracing the source of the quartz grains in sands and sandstones 

 Mackie (17) has advocated the use of inclusions found in the 

 quartz or their absence. He divides the inclusions into : 



(i) Regular inclusions, of quartz, chlorite, muscovite, bio- 

 tite, rutile, apatite, zircon, garnet, magnetite, titanifer- 

 ous iron, etc. Characteristic of the quartz of schists. 



(2) Acicular, or fine needle-like inclusions of doubtful min- 



eral, capable of being subdivided according to arrange- 

 ment of the needles. Characteristic of granites. 



(3) Irregular inclusions, mostly fluid lacunae with or with- 



out gas bubbles. Characteristic of granites, but more 

 readily subject to disruption by changes in tempera- 

 ture or to crushing than the others, and, hence, di- 

 minishing in proportion with the age of the forma- 

 tion, and the repeated reworkings of the grains. 



The absence of inclusions in quartz suggests vein quartz as the 

 source, or the quartz of schists. After a determination of the in- 

 clusions in the quartz grains of the crystallines which might have 

 furnished the sands of the Rothes Burn, and the determination of 

 the percentage of feldspar, garnet, staurolite, mica, hornblende, 

 chlorite and magnetite in the sands of this stream, together with 

 the inclusions in the quartz grains, Mackie concludes that 2^,% of 

 the sand was derived from granite, 57% from schist, and 20% 

 from diorite. The matrix of the Lower Old Red conglomerate of 

 Gollachy Mill was in like manner determined as derived 20% 

 from granite, y7% from schist, and 3% from diorite and volcanics. 

 In these analyses the feldspar was divided between the granite 

 schist and diorite, the garnet, staurolite, mica, chlorite and the 

 quartz with regular inclusions and without inclusions were referred 

 to schists, while the hornblende and magnetite were referred to 

 diorite. Analyses of the sand grains of successive members of 

 both the lower and upper Old Red Sandstone showed that, during 

 the process of erosion of the lands supplying the material of this 

 formation in northern Scotland, different sand-supplying forma- 



