4 Sir  william  Hamilton’s  Account  of 
high  rocks  of  lava  or  tuffa.  Where  the  volcanos  had  not 
operated,  the  mountains  and  rocks  are  of  Hate.  At 
Erpel,  in  a mountain  clofe  to  the  river,  and  oppofite  the 
convent  fituated  on  an  illand  about  three  leagues  from 
Bonn,  there  are  fome  traces  of  bafaltic  columns,  the 
quarry  feerning  to  have  been  nearly  exhaufted.  I have 
often  thought  (and  this  exhaufted  quarry  brings  it  to  my 
mind  again)  that  the  reafon  why  there  are  fcarcely  any 
remains  of  lavas  that  have  taken  the  columnal  form  on 
Vefuvius  and  the  volcanos  near  Naples  is,  that  they  have 
been  carried  off  for  the  ufe  of  paving  the  great  Roman 
roads.  The  Appian  way  is  moftly  compofed  of  lava  of  a. 
pentagonal  and  hexagonal  form,  and  feems  evidently 
made  of  pieces  of  fuch  bafaltic  columns.  Thefe  lavas 
being  ready  cut  by  nature,  would  naturally  be  carried  off 
firft,  as  the  cutting  of  folid  rocks  of  lava  for  fuch  pur- 
pofes  is  attended  with  very  great  expence. 
At  Unkel,  above  a league  further  on  towards  Coblenz, 
juft  oppofite  the  town  on  the  other  fide  of  the  Rhine,  is 
the  great  quarry  belonging  to  the  Eleftor  Palatine,  which 
affords  a moft  pleafing  and  uncommon  fight : it  is  entirely 
compofed  of  the  moft  regular  detached  bafaltic  columns, 
and  though  millions  of  thefe  columns  have  been  ex- 
tracted, as  the  towns  of  Cologne  and  Bonn  teftify,  yet 
the  quarry  is  very  rich..  They  lie  moftly  in  an  horizon- 
4 tal 
