126 REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 



H. Williams, in a recent paper on a siinilar outcrop in the 

 Cumberland Valley proposed the distinctive name of 

 " the ancient volcanic rocks " for this formation. 



OLIVINE ROCKS CONTAINING NO FELDSPATHIC CON- 

 STITUENT. 



No. 9. Serpen tine-Peridotites. 



This formation has its greatest development in the re- 

 gion of Newbury, two of the principal outcrops being in 

 localities popularly known as the Devil's den and the 

 Devil's basin. They are represented on the map by num- 

 bers on the outcrops. Since the area was mapped, micro- 

 scopic studies of thin sections of these rocks have proved 

 the presence of much augite and hornblende with the Ser- 

 pentine surrounding them, and also developed in the 

 cleavage cracks of these minerals, thus proving that the 

 rock-mass was originally an augite-hornblende-picrite- 

 peridotite. The outcrops in Lynnfield, Peabody and 

 Boxford while probably of the same character have not at 

 present been absolutely proved as such. Sections studied 

 thus far are composed of Serpentine and magnetite with 

 colorite and other minerals. 



No. 10. Biotite-Mica-Peridotite. 



This rock-mass appears on the banks of the Skug river 

 in Andover. When studied from thin sections it is seen 

 to be composed of biotite-mica which is bleached to a 

 nearly white color, calcite, talc, Serpentine and mag- 

 netite surrounding irregulär patches of olivine which is 

 rare, some tremolite and a few small masses of augite 

 which is also surrounded by Serpentine. 



