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 1 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 quartz iferous 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 Oleuellus, Lower Cambrian. Mr.. Auguste 

 F. Foerste first described one of these fossils (Hyolithes 

 incequilateralis, 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 Hyolithes princeps, Hyolithes communis, var. 

 emonsii, Hyolithes impar and Stenotheca rugosa; all of 



1 A paper on the Geology of Nahant by A. C. Lane, will be found in the Proc. 

 Boston Soc. Nat. Hist., Vol. xxiv, p. 91. 



