56(3  PRE-CAMBRlAN    GEOLOGY   OP   NORTH   AMERICA. 
schistosity  and  stratification  dip  eastward,  but  the  exact  angle  at 
which  the  strata  lie  is  difficult  of  determination.  Localities  on  the 
western  border  have  a  steep  westerly  dip  in  many  instances ;  in  others 
the  border  series  as  a  whole  is  in  a  nearly  vertical  position.  The 
orographic  thrust  producing  the  folding  was  directed  nearly  east  and 
west.  Normal  faults  and  overthrusts  are  indicated,  but  data  for  their 
detection  are  not  now  at  hand  except  in  one  instance. 
Whittle,33  in  1894,  describes  the  pre-Cambrian  rocks  of  Vermont 
as  consisting  of  two  series  of  Algonkian  rocks.  The  Lower  Cam- 
brian quartzite  is  apparently  underlain  conformably  by  the  upper  of 
the  two,  or  the  Mendon  series.  That  the  two  are,  however,  really 
unconformable  is  supported  by  the  following  reasons  among  others: 
The  extreme  lithological  diversity  of  the  metamorphic  series  as  com- 
pared with  the  quartzite;  a  close  folding  in  the  Mendon  series  not 
observed  in  the  quartzite,  and  the  fact  that  toward  the  south  the 
quartzite  reposes  discordantly  upon  granitoid  gneiss. 
The  Mendon  series  consists  in  descending  order  of  the  following 
members:  Mica  schists,  with  a  maximum  thickness  of  1,000  feet; 
micaceous  quartzite,  having  a  maximum  thickness  of  500  feet  and 
carrying  several  thin  beds  of  crystalline  limestone;  crystalline  lime- 
stone, with  a  maximum  thickness  of  400  feet;  conglomerate  schists 
and  quartzite,  with  a  maximum  thickness  of  700  feet.  At  Mendon 
the  section  has  an  approximate  thickness  of  1,300  feet,  and  in  some 
localities  there  may  be  2,000  feet  of  strata  in  the  series. 
The  lower  series  of  Algonkian,  called  the  Mount  Holly  series,  is 
contrasted  with  the  Mendon  series  in  nearly  every  way.  The  struc- 
ture of  the  series  is  so  complicated,  the  different  rock  types  vary  so 
greatly,  and  the  series  has  been  subjected  to  such  a  multiplicity  of 
dynamic  movements,  that  no  definite  stratigraphy  has  been  made  out. 
Some  of  the  prominent  rocks  of  the  series  are  biotite  schist,  muscovite 
schist,  garnetiferous  schist,  vitreous  quartzite,  augen  gneiss,  and  vari- 
ous kinds  of  limestone.  The  limestones  are  in  irregular  lenses  and 
are  extremely  local.  There  may  be  two  horizons  of  limestone  or  a 
dozen.  The  series,  because  of  the  undoubted  areas  of  sedimentary 
rocks  which  have  escaped  destruction,  are  regarded  as  clastic.  Asso- 
ciated with  the  above  rocks  are  very  abundant  schistose  igneous  rocks, 
comprising  both  dikes  and  sheets. 
The  two  series  of  Algonkian  rocks  are  regarded  as  unconformable 
for  the  following  reasons :  Between  the  two  there  is  a  great  litholog- 
ical difference ;  the  Mount  Holly  series  has  been  cut  through  by  erup- 
tive rocks  in  a  complicated  fashion,  and  these  do  not  occur  in  the 
Mendon  series;  the  Mount  Holly  series  is  folded  in  a  much  more 
intricate  manner  than  the  Mendon  series,  and  secondary  structures 
have  developed  to  a  far  greater  degree ;  at  the  bottom  of  the  Mount 
Holly  series  is  a  widespread  formation  of  conglomerates  and  gneiss. 
