Brohl  (p.  61 ;  184  ft.  above  the  sea)  lies  at  the  mouth  of  the
Brohlthal,  a  deep  winding  valley,  enclosed  by  wooded  moun-
tains, and  enlivened  with  numerous  mills  and  houses.  An  object
of  great  interest  is  the  extensive  stratum  of  Tufa,  50-100  ft.  in
thickness ,  of  which  the  floor  of  the  valley  consists ,  and  above
which  rise  slopes  of  Devonian  slate  (p.  xviii).  This  tufa  is  an
important  article  of  commerce,  and  Is  extensively  excavated  from
quarries  and  mines  on  both  sides  of  the  valley.  When  pounded
('trass')  and  mixed  with  lime  it  possesses  the  invaluable  property
of  hardening  under  water,  and  is  largely  exported  to  Holland  for
the  construction  of  dykes.  Numerous  springs  strongly  impregnated
with  carbonio  acid  gas,  near  the  Laacher  See,  and  especially  in  the
Brohlthal,  still  afford  indication  of  Blumbering  volcanic  agency,


88     Route  15.  LAACHER  SEE.
The  road  which  ascends  the  valley,  skirting  the  Brohlbach,  is
flanked  on  both  sides  with  numerous  tufa  quarries,  some  of  which
are  open,  while  others  are  driven  like  mining-shafts  into  the  hill.
After  2  M. ,  in  the  middle  of  the  valley ,  rises  the  small  castle  of
Schweppenburg  (311  ft.),  probably  erected  in  the  16th  century.
The  garden  contains  a  Roman  altar  found  here.
The  Heilbbunneb  Thai.,  which  diverges  here  to  the  S.,  contains  the
Heilbrunnen  (384  ft),  a  mineral  spring  of  saltish,  hut  refreshing  taste,  sim-
ilar to  the  Kreuzbrunnen  of  Harienbad.
