80 Prescolt's Sketch of the Geology and Mineralogy 



so completely disintegrated as to be converted into a perfectly 

 friable earth, or soil, of a dark brown color. This gives to these 

 veins a singular appearance and renders them worthy the atten- 

 tion and inspection of the Geologist. 



The north eastern section of Nahant, is also composed of 

 sienite. The west part of Nahant, called Bass Point, is com- 

 posed of a different species of sienite, the hornblende and feld- 

 spar being in nearly equal proportions. 



At the south east part of Little Nahant, occurs a singular 

 rock, composed of hornblende, and a dark reddish brown feld- 

 spar, and may be classed as a variety of sienite. 



In general sienite passes into greenstone, and often by insen- 

 sible shades, and greenstone may be seen protruding above the 

 surface, near the base of all the hills of sienite, and often crop- 

 ping out at considerable elevations. The same occurs among 

 all the porphyry hills in Lynn and Saugus. 



Although bowlders of the Danvers sienite are never seen 

 north of the bed, yet at the south, they occur scattered over the 

 greenstone ridges of Salem and Marblehead, and over the por- 

 phyry ridges of Lynn and Saugus. Many are found on Nahant 

 and a large number occurs on Little Nahant. These bowlders 

 resemble the Danvers sienite, and differ from any found in the 

 vicinity. 



A rock, somewhat different from any other in its composition, 

 but which may be classed with the sienite rocks, occurs in the 

 west part of Topsfield, and the east part of Middleton, near their 

 junction with the north part of Danvers. This rock is com- 

 posed of red feldspar, quartz and green chlorite ; the chlorite 

 taking the place of hornblende. In passing north through Dan- 

 vers, you meet with small bowlders of this rock, several miles 

 before reaching the north line of the town, and as you advance 

 north they increase in size and frequency ; until (within a mile 

 or a mile and a half of the ledge) the stone walls are chiefly 

 composed of it. This ledge or parent rock, you will find in 

 place on both sides of Nichols' Brook, in the corner of Middle- 

 ton and Topsfield. The general aspect of this rock, from the 

 red feldspar, which is the most abundant ingredient, will very 

 readily distinguish it from any other in this section of the county. 



