32 THE STRATIFIED ROCKS OF ESSEX COUNTY. 
color, obliteration of fossils, and crystallization, with or 
without change in the constituent minerals of the rock. As 
examples ; by metamorphism, zoisite, glaucophane, chlo- 
rite, leucoxene, epidote, etc., are formed as new minerals, 
while secondary formations of quartz, glassy feldspar, cal- 
cite and epidote are often seen. Brown hornblende is 
altered to green hornblende, augite to hornblende and 
magnetite, biotite to green chlorite and magnetite, besides 
other important changes. 
Among the most interesting of the stratified rocks are 
the Nahant^ limestones ; they are first seen on the south 
side of Nahant Head at the Shag rocks and extend about 
three hundred yards to a point just beyond Bennett's Head 
on the north. The limestones are much metamorphosed in- 
to bands of light and dark lydite, microscopic sections of 
which reveal calcite, quartz grains, magnetite and mica, 
with occasional masses of nearly pure calcite interstratified 
with an indurated quartziferous slate. In thin sections un- 
der the microscope they are shown to be composed of calcite, 
epidote, quartz, serpentine, white garnets and limonite ; 
chlorite tinges portions of the rock green, while hematite 
and limonite turn other parts red, thus giving the mass a 
brightly banded appearance, its most striking feature to 
casual visitors. By means of certain fossils which have been 
found in this rock the horizon of its formation is deter- 
mined as the Olenellus, Lower Cambrian. Mr. Auguste 
F. Foerste first described one of these fossils {Hyolithes 
inmquilateralis, sp. nov.) in the Proc. of the Boston So- 
ciety of Natural History, Vol. 24, p. 262, and I have since 
collected from the region numerous specimens of this species 
and also of HyoUUies princ€X)S, HyoUlhes communis^ var. 
emonsii, Hyolilltes impar and Sleuolheca rugosa; all of 
' A iiaper on the Geology of Nuliaut by A. C. Lane, will be fouiul iu the Proc. 
BoBlou Soc. Nat. Hist., Vol. xxiv, p. 91. 
