124 
ME.  J.  H.  COOKE  OK  THE  MARLS  AND 
[Feb.  1893, 
gradual  character.  This  line  of  separation  is  by  no  means  uniformly 
horizontal,  and  it  takes  an  undulatory  form.  Both  the  Clays  and 
the  Greensands,  therefore,  vary  greatly  in  their  thickness,  and  it 
often  happens,  as  at  Dingli,  -where  the  Greensands  are  50  feet  and 
the  Clays  only  6  or  10  feet  thick,  that  the  maximum  development 
of  the  Greensands  is  accompanied  by  a  minimum  thickness  of  the 
Clays.  This  undulatory  surface  of  the  Clays  forms  a  series  of 
natural  reservoirs  in  which  the  rains  of  winter  are  stored,  and  it  is 
from  these  that  the  population  of  the  Islands  derive  their  water- 
supply. 
III.  Lithological  and  Mineralqgical  Characters 
of  the  Strata. 
A  microscopic  examination  of  numerous  sections  that  had  been 
taken  from  various  horizons 1  of  the  Clay-formation  revealed  the 
general  structure  of  the  rock,  and  showed  it  to  consist  of  tests  of 
foraminifera  and  minute  fragments  of  minerals,  the  most  numerous 
of  which  were  oxide  of  iron  and  glauconite.  Bounded  grains  of 
quartz,  augite,  hornblende,  felspars,  zircon,  and  tourmaline 2  were 
also  present  in  comparative  abundance  in  every  part  of  the  Clay, 
but  more  especially  so  in  the  upper  portions  near  the  junction  with 
the  Greensands. 
Sulphate  of  lime  in  the  form  of  selenite  occurs  also  in  consider¬ 
able  quantities.  It  is  generally  found  in  the  Clay  in  isolated 
masses  of  varying  size  and  shape,  or  in  aggregations  of  small 
crystals  that  have  grown  round  organic  remains :  these  crystalline 
masses  usually  present  a  very  lamellar  or  fibrous  appearance.  As  a 
fertilizing  agent  their  advantages  are  fully  appreciated  by  the  Maltese 
agriculturist,  and  it  is  partly  to  the  abundance  of  selenite  in  the 
Marls  and  Clays  that  Malta  and  Gozo  owe  their  luxuriant  crops  of 
clover  and  cereals. 
A  characteristic  of  the  upper  portions  of  the  Clay-formation  is 
the  presence  of  yellow,  ochreous  nodules  of  clay,  the  majority  of 
which  are  ovoid  or  subspherical  in  shape  :  they  are  generally  much 
flattened  above  and  below.  When  these  nodules  are  split  along 
their  bedding-planes  the  perforations  of  Pholades  are  revealed,  and 
often  the  casts  of  the  Pholades  themselves  are  found  within. 
Similar  nodules,  consisting  of  indurated  blue  clay,  are  equally  abun¬ 
dant  in  the  lower  divisions  of  the  Clay.  In  both  varieties  the 
laminae  show  distinct  evidences  of  stratification,  and  they  always 
occur  lying  parallel  to  the  flattened  surfaces  of  the  nodules. 
Prof.  Leith  Adams  alludes  to  these  nodules  as  being  derivative 3 ; 
1  Besides  the  many  slides  that  I  prepared  myself,  I  was  enabled,  through  the 
courtesy  of  Dr.  John  Murray,  to  examine  the  numerous  sections  which  he  had 
caused  to  be  prepared,  and  which  he  subsequently  described. 
2  Dr.  John  Murray,  ‘The  Maltese  Islands,  etc.,’  Scot.  Geogr.  Mag.  yol.  vi. 
(1890)  p.  449. 
3  Quart.  Journ.  Geol.  Soc.  vol.  xxxy.  (1879)  p.  519 ;  and  ‘Botes  of  a  Natu¬ 
ralist  in  the  Bile  Valley  and  Malt?,’  Edinburgh,  1870,  p.  131. 
