12 GUIDH TO LOCALITIES. 



tective effects of algae and barnacles are displayed to great advan- 

 tage on the broad rock shelf which extends from the eastern end 

 at tide level. 



Between Little and Great Nahant is a fine wall beach, high and 

 coarse at each end. The gradation from angular blocks to well- 

 worn pebbles can be followed in all its stages. At the north end is 

 a partial dead-water, wherein great masses of kelp lodge. On the 

 back side of the beach many angular pieces may be found, thrown 

 up chiefly during the storms of winter. At times in this season 

 the road is impassable. It is probable that the " pocketing " of 

 the material is due, not to currents from either end, but to the 

 greater blow and transporting power of long waves on the two 

 horns, and the rapid loss of this power as the wave moves toward 

 the apex of the angle. 



Great Nahant is composed of two islands, and the lines of union 

 which have made them one can be seen clearly. The swampy 

 area ruuning south between the main hills and Bear pond marks 

 the position of the old strait. The main igneous rock is a coarse 

 diabase. On the south and southeast are lower Cambrian sedi- 

 ments. They consist chiefly of indurated pelite and white limestone ; 

 the former containing calcareous concretions, which are repre- 

 sented largely by cavities filled with epidote crystals. They are 

 more strongly marked below the sheets of diabase, and become 

 less frequent farther away. 



The limestone is fossiliferous, bearing indistinct outlines chiefly 

 of JEyolithes communis var. emmonsi. The strike of the series is 

 N. 50° E., its dip about 45° N. W. Trap dikes intersect both 

 sediments and diabase. Just beyond the outcrop of the white 

 limestone beds a transverse fault may be seen. The beds on the 

 north have been thrown down thirty or more feet, bringing the 

 white limestone near sea level. The main trap sheet, which on 

 the south of the fault is exposed in the field south of the stone 

 building, north of the fault outcrops along the cliff walk. Below 

 it is the best place to observe the altered calcareous concretions. 

 The fault line is marked by a fine chasm east of the cliff walk. 



The difference in effect of the marine action upon the igneous 

 rocks and elastics is brought out clearly here. The former have 

 the rounded outlines seen elsewhere on the coast. The latter are 

 angular, bold, rugged. Several systems of joints intersect the 



