18 



• The same feldspar, apparently, occurs on Salem Neck, and 

 at several other points in that region. In a formation so desti- 

 tute of free silica as the Naugus Head series, it were natural to 

 expect to find the feldspar mainly triclinic ; and this expecta- 

 tion is fully realized, for, save in the very coarse feldspar rock 

 of the preceding paragraph, I have rarely failed to observe the 

 striae indicative of plagioclase ; and in not a few instances, judg- 

 ing by physical characters alone, this plagioclastic feldspar is 

 most probably labradorite. A triclinic feldspar from the west 

 end of Salem Neck, apparently little altered, afforded Mr. 

 Geo. H. Barton fifty-seven per cent, of silica. 



Dark-colored mica, probably biotite, is common in the rocks 

 of this formation, though seldom abundant. Pyroxene appears 

 as a principal constituent, and hypersthene is believed to occur. 

 The massive, coarsely crystalline diabase, or norite, at Nahant 

 is often decidedly epidotic ; and the epidote is particularly 

 abundant on the south shore, east of the steamboat wharf. 

 The most striking, and probably the most important minera- 

 logical character of this series of rocks is, that all members of 

 it are absolutely destitute of quartz. Lithologists will recognize, 

 in the complete absence of quartz from this formation, a strong 

 indication that the principal constituent, after feldspar, is 

 pyroxene, and not hornblende ; and it is probable that every 

 basic rock of this series may be properly classified, in a gen- 

 eral way at least, as either diabase, norite, or pyroxenite. 

 The coarse feldspar rock, mentioned above, and consisting 

 mainly of orthoclase, is probably often a true syenite. 



That this series of pyroxenic and feldspathic rocks, with its 

 •associated minerals, — which is sometimes stratified, oftener 

 eruptive, frequently very coarsely crystalline, and always 

 quartzless, — is distinct from anything observed elsewhere in 

 Massachusetts, cannot be doubted. Lithologically at least, it 

 may be said to be sui generis in Massachusetts geology. The 

 great disturbance which the Naugus Head series everywhere 

 exhibits, and its thoroughly crystalline appearance, stamp it as 

 older than the Huronian and Montalban formations ; and other 



