J. T. Jut son — Rounding of Pebbles. 431 



ately adjacent. Moreover, rocks, rounded at the surface, 

 may be seen breaking away from the parent mass. 



(2) — The greenstone pebbles are scattered at all 

 heights from the bottoms to the tops of the hills. This 

 fact, therefore, precludes the action of lake waters and 

 also of stream action, unless, in the latter case, the peb- 

 bles were laid down on the tops of the hills (when the 

 general land surface stood approximately at the height 

 of such hills) and have been subsequently transported by 

 gravitation down their sides. The close proximity of 

 the parent rock to the pebbles, however, is against this 

 possibility. 



(3) — Strong water abrasion would reduce most of the 

 soft sediments and porphyries to powder rather than 

 form rounded pebbles. 



A third hypothesis is spheroidal weathering, which, 

 although operating among the greenstones by flaking, as 

 described below, does not appear to meet all the facts. 

 No actual peeling of concentric coats, in either of the 

 classes of pebbles described, has been seen; and the 

 stratified sediments and decomposed foliated porphyries 

 are not favorable rocks for this mode of weathering. 



A fourth hypothesis that they are derived from old 

 conglomerates is equally untenable in view of their clear 

 relationship to the rocks in situ, as already shown. 



In the greenstones, the iron crust or film on the joint 

 faces is important. Until broken by ordinary weather- 

 ing agencies no rounding, as a rule, takes place, but once 

 the crust is penetrated it tends to be slowly removed 

 over the whole surface, and the grey rock within begins 

 to assume a rounded form. Examples are obtainable of 

 various stages of this destruction of the crust. An iron 

 crust does not cover the sediments and porphyries ob- 

 served, hence the process of pebble formation is not 

 delayed for a time as in the greenstones. 



Flaking in small fragments, owing to temperature 

 variations, may and probably does take place in the 

 rocks of both classes of pebbles ; but in the case of the 

 greenstones practically only when the crust has been 

 broken. This flaking may be regarded as a phase of 

 spheroidal weathering, and doubtless is partly responsi- 

 ble for the greenstone pebbles. The chief agent, how- 

 ever, of the rounding of the other class of rocks, and a 

 major or minor agent in the formation of the greenstone 



