CLAYS  OF  PENOBSCOT  BAY  REGION,  MAINE.        431 
at  the  village  of  South  Thomaston  (4  miles  south  of  Rockland).  There  has  recently  been 
some  talk  of  utilizing  the  power  at  this  point.  Marine  clays  of  fine  and  uniform  texture 
occur  abundantly  in  this  vicinity,  the  clay  of  analysis  No.  2  being  taken  from  Hayden 
Point,  only  1  mile  distant. 
Possible  utilization  in  the  manufacture  of  Portland  cement. — The  lime  industry  of  the  Rock- 
land region  is  discussed  by  the  writer  in  another  part  of  this  bulletin  (pp.  393-400),  and  it  is 
worth  considering  whether  the  marine  clays  may  not  be  utilized  with  this  limestone  in  the 
manufacture  of  Portland  cement.  These  cements  are  artificial  mixtures  whose  essential 
constituents  are  lime,  silica,  and  alumina.  The  first  is  generally  supplied  by  limestone  or 
marl,  the  other  two  by  clay.  In  burning,  the  three  constituents  unite  to  form  complex 
silicates,  and  it  is  essential  that  they  be  combined  in  the  proper  proportions  in  order  to 
give  the  best  results. 
In  clays  utilized  in  the  manufacture  of  Portland  cement  the  silica  percentage  should  lie 
between  60  and  70  per  cent.  These  clays  show  62  to  63  per  cent  silica.  According  to 
Eckel,  "the  alumina  and  iron  oxide  together  should  not  amount  to  more  than  one-half  the 
percentage  of  silica,  and  the  composition  will  usually  be  better  the  nearer  the  ratio  A1203-|- 
Fe.,():i=Si02."«  In  the  clay  from  Hayden  Point  (No.  2  in  the  table)  this  ratio  is  Si02.. 
3  ~2T 
In  reference  to  the  quantity  of  clay  needed  Eckel  b  estimates  that  there  should  be  in  sight 
at  least  1,600,000  cubic  feet  of  clays,  a  twenty  years'  supply.  This  would  mean  only  3.67 
acres  excavated  to  the  moderate  depth  of  10  feet.  In  view  of  what  has  been  said  of  the 
extent  and  depth  of  the  clays  there  should  be  no  doubt  as  to  the  adequacy  of  the  supply. 
The  limestone  used  should  be  relatively  free  from  magnesia.  Lime  made  from  a  highly 
magnesian  limestone  has  the  property  of  setting  under  water  to  a  very  hard  mass.  When 
mixed  with  clay,  however,  and  burned  at  high  temperatures  in  a  Portland  cement,  it  gives 
cements  of  doubtful  character.  The  reason  for  this  is  that  the  carbonate  of  magnesia, 
unlike  the  carbonate  of  lime,  does  not  ordinarily  combine  with  silica  or  alumina  at  the 
clinkering  heat  employed  in  the  manufacture  of  Portland  cement.  In  amounts  of  less  than 
4  or  5  per  cent,  however,  magnesia  is  certainly  not  injurious.  The  so-called  "soft  rock" 
of  this  region,  being  poorest  in  magnesia,  would  be  the  most  available  for  cement  purposes. 
As  regards  quantity  needed,  Eckel  &  estimates  that  at  least  3,800,000  cubic  feet  should  be 
in  sight  for  each  kiln  established,  a  twenty  years'  supply.  This  would  correspond  to  a  vein 
of  rock  100  feet  wide,  100  feet  deep,  and  380  feet  long.  The  amount  of  limestone  in  the 
region  is  probably  adequate  to  meet  these  demands. 
At  present  the  nearest  Portland-cement  works  are  located  in  the  eastern  part  of  New 
York  State,  and  the  opportunities  for  building  up  a  good  local  cement  market  seem  to  be 
good.  With  the  same  advantages  of  easy  and  cheap  shipment  by  water,  there  seems  to 
be  no  reason  why  Rockland  cement,  like  Rockland  lime,  should  not  be  able  to  hold  its 
own  in  the  New  York  market,  as  well  as  at  other  points  on  the  Atlantic  coast.  In  this 
connection  it  may  be  borne  in  mind  that  nowhere  on  this  coast  south  of  Rockland  do  pure 
low-magnesia  limestones  occur  near  the  seaboard. 
a  Eckel.  E.  C.  Cements,  Limes,  and  Plasters,  1905,  p.  354. 
b  Op.  cit.,  p.  305. 
