Lane.] 
92 
[Dec. 5 , 
is probably due to the resorption of olivine by a residual magma 
containing potash, which thus formed biotite instead of or in addi- 
tion to augite, as described by Rosenbusch . 1 The analysis of the 
Medford diabase, which, as described by Hobbs , 2 resembles that of 
Nahant in many ways, strengthens this supposition. 
Along slickensides and veins and where the diabase is crushed, 
prehnite is developed. The intertwined structure described by 
Bentell 3 and Mallard 4 is often seen in thin section. The ready 
fusibility of prehnite to a blebby glass distinguishes it from feld- 
spar. It is generally in radiating groups. 
Near Saunder’s ledge the rock is puzzling, being differentiated 
into more and less basic portions mutually inclusive. Occasionally 
feldspar is developed here in crystalline form. 
The syenite of Little Nahant, although in many ways akin to the 
main mass, contains orthoclase and, microscopically, quartz as well 
as microliths of zirkon. 
The brightly banded altered sediments at the east end of the 
peninsula, from Bennett’s Head to the Shag Rocks, are the most 
striking feature to visitors. 
The darker bands are a lydite or indurated quartziferous slate, 
in which the microscope reveals, besides a ground mass of minute 
quartz grains, rusty flecks of ilmenite and mica. 
The lighter bands are largely made of microscopic garnets, to 
which calcite, epidote, quartz and other minerals of less importance 
are added. Chlorite tinges the rock green, hematite turns it red. 
The black lydite is a good touchstone. The lighter bands take the 
streak of minerals well. Both are good whetstones when of suffi- 
cient size and of even grain. The light bands are much like the 
Belgian whetstone. They are normally irregularly tubular or nod- 
ular, but are often confluent. 
Small beds of diabase are intercalated in the sediments, which 
are not of themselves sufficient to produce such alteration as de- 
scribed. This is due to the great diabase sheet, whose upper limit 
is not known (the well at Mr. Schlesinger’s shows that it is over 
two hundred feet thick). Under this sheet the sediments dip in 
every case, — at Nahant Head and also at Black Rock and John’s 
Peril, where are also obscure outcrops. Between Dorothy’s Cove 
and Pond Beach, boulders of nodular slate are found, the origin 
1 Mik. Physiog., II, 485. 2 Bull. Mus. Comp. Zool., 1888. 
8 Neues Jahrbuch, 1887, p. 89. 4 Bulletin Soc. Min. Francais, 1882, p. 195. 
