98 OUTI.IM'N .i FIBLD-GBQLO 



geologist will, of course, be guided by the local circum- 

 stances of each case. For example, if the surface of 

 the ground should present many rounded pebbles and 

 boulders, he will not at once conclude that these frag- 

 ments have been derived from solid rock below, 

 rounded forms will rather raise a suspicion that 



been transported, and should many of them plainly 

 show the characteristic smoothed surface of water -\\orn 

 stones, they will be set down as derived immediately from 

 some adjacent bed of gravel or conglomerate. The mere 

 fact of a great variety of separate rounded rock- fragments 

 occurring over the surface at any locality, suggests a mass 

 of transported material, rather than the decomposition of 

 the solid rock underneath. 



On the other hand, the occurrence of abundant angular 

 fragments of rock on the surface, at once arrests attention, 

 as indicative of the vicinity of that rock /// situ. The 

 observer travetses the ground in all directions in search 

 of any projecting knob of the actual rock itself. Failing 

 to find it, he notes the position of these angular chips, 

 and tries whether they can be traced further, so as to 

 indicate by their distribution at the surface the probable 

 trend of the solid rock underneath. In ascending a hill- 

 side so covered with trains of detritus or vegetation that 

 no rock can be seen in place, the geologist may learn 

 much regarding the concealed rocks by examining the 

 debris. He knows that the fragments of stone have 

 rolled down, and not up. When, therefore, in his ascent, 

 he observes that the angular chips of some particular 

 rock, abundant enough below, no longer appear, he sur- 

 mises that he must have crossed the limits of the solid 



