1893.] evidence for the Recurrence of Ice Ages. 101 



that specimen or it would have been worn down to an even sur- 

 face. On the principle of " diamond cut diamond " the hardest 

 white quartz is sometimes polished and striated as in No. 7 : a 

 similar kind of wear being seen on the face of the siliceous Burr- 

 stone which has been used for any length of time for grinding in a 

 mill ; see No. 8. 



Fragments of a rock-face smoothed and fluted by glacial action 

 seem to have been not uncommonly detached by denudation from 

 the boulder or parent rock and entombed in newer deposits. This 

 takes place now where cliffs of boulder clay are exposed to the 

 action of frost and sun. The included masses of rock break up 

 under their influence and, without suffering much wear and tear, 

 get buried in the clayey mass into which they fall. The grooves 

 on such fragments are cut off with their full breadth at the margin 

 of the stone whereas, if the striations had been produced on the 

 stone as it is, the thin edge would have broken away. An 

 example of such a broken boulder is seen in No. 3 or better still 

 in No. 4. 



iv. Stones natm-ally smoothed and polished hut not hy glacial 



action. 



In earth movements when the parts of a fissured rock are 

 relatively displaced and the sides of a fault are dragged against one 

 another, perhaps even with a to-and-fro motion, if the cut be clean 

 and the rock of a somewhat homogeneous texture, it will be 

 smoothed and polished. Minerals commonly form along the crack 

 or the exterior of the rock-faces in the fault undergoes mineral 

 change. This is a very important character in helping us to dis- 

 tinguish between one of these slickensides as they are called and 

 the smoothed face produced by any surface action. In this way a 

 rock as rough as or rougher than the Molasse (see No. 1 ) may have 

 a soft sheen given to it, as in the case of some specimens from the 

 New Red Sandstone, see No. 9, on which among other minerals 

 carbonate of copper has been formed, or No. 10, on the face of 

 which a film of mineral matter has produced a surface like a 

 mirror. 



Some minerals and rocks which are readily soluble in water, 

 either pure or charged with a small quantity of such acids as are 

 commonly present in surface waters, have a polish produced by the 

 chemical decomposition of their surface. This often happens to 

 masses of carbonate of lime or limestone exposed to the spray 

 from a waterfall or embedded in clay. Nos. 11 to 13 are examples 

 of this process. 



Rocks polished in this way are generally fretted into irregular 

 pits as shown on the fragment of limestone No. 14 and more 

 obviously on the piece of gypsum No. 15. 



