The Rocks of the John Day Valley. 167 



brought to a close. The last rock of the 

 series fills the place of a cover to the volume. 

 Never was cover better defined nor more dis- 

 tinctly separated from the well written and 

 well illustrated pages it serves to protect. The 

 cover itself, too, has a history worth reading. 

 It extends for miles, varying but slightly in 

 thickness, which amounts to twenty or 

 twenty-five feet, and is throughout so entirely 

 volcanic as to leave no room for mistake. Its 

 materials -are volcanic ashes and cinders, the 

 cinders ranging from an inch across down- 

 ward to the minuteness of the ashes. One 

 can hardly look at a piece of this rock without 

 recalling the younger Pliny's vivid description 

 of the shower of cinders from Mount Vesu- 

 vius, from which he saw people escaping with 

 pillows tied on their heads for protection. 

 Such showers fell here certainly over hun- 

 dreds of square miles and in such vast bulk 

 that, pressed by the hydraulic force of later 

 masses above it into a solid plate of rock, it 

 now in this form measures from twenty to 

 twenty-five feet through. No wonder it closed 



