A PECULIAR CLEAVAGE STRUCTURE 



557 



examples of the "pebbles" are composed chiefly of quartz, except- 

 ing those produced from vein quartz to be described directly. 



Traversing the country rock in all directions are quartz veins, 

 true chemical segregations. Though formed subsequently to the 

 rock in which they are found, they antedate, in this restricted locality, 

 the period of metamorphism in which the pebbles described above 

 were produced, for the 

 reason that the structure 

 has been produced in the 

 quartz vein itself. Indeed 

 it is from such material 

 that the most perfect 

 "pebbles" were obtained 

 (see Fig. i). The quartz 

 veins in the railroad cut 

 vary from a fraction of an 

 inch up to 6 inches or more 

 in thickness. Toward the 

 northwest end of the cut 

 under discussion, and just 

 before the gradual curve 

 to the right begins, will 

 be found the best exhibi- 

 tion of the "pebbles." In 

 the fall of 1907 one of the 



quartz veins could be traced fairly well across the cut, which at 

 the time was thought to have furnished the bulk if not all of the 

 best specimens. This evidence, however, is not at all conclusive 

 at the present time, for on a more recent visit in the fall of 1909, 

 this tracing could not be made at all, on account of slides in the 

 clay in the cut. That the vein quartz has furnished the material 

 for many of the lenses of "pebbles," however, can be shown from the 

 specimens collected at the earlier visit, in which the gradation from 

 the latter into the former is well brought out (see Figs. 2 and 3). 

 In such hand specimens the quartz lenses may be picked out and 

 replaced in their former positions, the whole appearing as a single 

 solid mass when bound with an elastic band. Specimens were col- 



FiG. I. — A group of quartz lenses, resulting 

 from pressure on vein quartz. EUijay, Gilmer 

 County, Georgia. 



