These  tuffs  of  the  southern  shores  of  the  Grand  Canary,  con-
taining the  Upper  Miocene  shells,  appear  to  be  about  the  same
age  as  the  most  ancient  volcanic  rocks  of  the  island,  composed
of  slaty  diabase,  phonolite,  and  trachyte.  Over  the  marine
lavas  and  tuffs  trachytic  and  basaltic  products  of  subaerial
volcanic  origin,  between  4,000  and  5,000  feet  in  thickness,  have
been  piled,  the  central  parts  of  the  Grand  Canary  reaching  the
height  of  about  6,000  feet  above  the  level  of  the  sea.  A  large
portion  of  this  mass  is  of  Pliocene  date,  and  some  of  the  latest
lavas  have  been  poured  out  since  the  time  when  the  valleys
were  already  excavated  to  within  a  few  feet  of  their  present
depth.
On  the  whole  the  rocks  of  the  Grand  Canary,  an  island  of  a
nearly  circular  shape,  and  6|  geographical  miles  diameter,
exhibit  proofs  of  a  long  series  of  eruptions  beginning  like  those
of  Madeira,  Porto  Santo,  and  the  Azores,  in  the  Upper  Miocene
period,  and  continued  to  the  Post-pliocene.  The  building  up
of  the  Grand  Canary  by  subaerial  eruptions,  several  thousand
feet  thick,  went  on  simultaneously  with  the  gradual  upheaval  of
the  earliest  products  of  submarine  eruptions,  in  the  same  man-
ner as  the  Pliocene  marine  strata  of  the  oldest  parts  of  Vesuvius
and  Etna  have  been  upraised  during  eruptions  of  Post-tertiary
date.
