202 OUTI.IM -;s ui 1 IKLD-GEOLor.Y PARTI 



have been carried by ice to their present positions. 

 Before accepting this interpretation, however, the ob- 

 server should endeavour to ascertain whether they are 

 really of a material foreign to the district. In many cases, 

 he will find that they are not ; that on the contrary, their 

 parent rocks, or at least rocks having precisely the same 

 lithological characters, lie near at hand. Thus, the well- 

 known Druid or Sarsen stones, so abundantly strewn over 

 the plains of Wiltshire, were formerly supposed to have 

 been carried from some extraneous source, but they are 

 now recognised as fragments from a sandy stratum which 

 once covered that part of the country. It is surprising 

 over what a slight inclination large blocks may slowly 

 move in the course of years, as the soil underneath is 

 mixed by worms and roots, and gradually shifted towards 

 lower levels. Sometimes escarpments which once sup- 

 plied a crop of blocks to the slopes below, get gradually 

 buried under their own debris, and are in the end earthed 

 and grassed over, so that those blocks which may still 

 remain exposed, and have survived the crags that sup- 

 plied them, might be taken for far-transported erratics. 

 It is essential, therefore, when the observer wishes to 

 determine beyond question whether a particular boulder 

 is due merely to the disintegration of a rock in situ or to 

 transport from some distance, first of all to make sure 

 that there is no parent rock in the immediate vicinity to 

 which the rock can possibly be referred, either because 

 its composition is different, or because its position shows 

 that it could not have come by mere ordinary decay and 

 removal. If he can prove that the block is foreign to 

 the district, or that though rocks of the same character 



