Wadsworth.] 
288 
[November 2, 
earth’s crust by the shrinking of the interior, are by far the most 
important ; for to this division are to be referred all folds, faults, 
slaty cleavage and most kinds, at least, of joint structure. The 
chemical forces, on the other hand, although all important so far 
as changes in the composition and texture of rocks are concerned, 
produce only some very subordinate structural features, such 
as concretions, stalactites, caverns, etc. 
The superficial agencies involved in the formation of rock- 
structures are chiefly air, water, and, to a very limited extent, 
animals and plants ; in short — the agents of erosion ; and these 
derive their energy almost entirely from the solar radiation. 
Erosion, it is well known, is both mechanical and chemical, but 
the two kinds usually go on together, so that it is scarcely practi- 
cable to classify the structural features produced by the superficial 
agencies in this way. But we can properly and easily distinguish : 
(1) the common forms of relief produced by moving water and 
air aided by chemical decomposition, as — mountains, hills, ridges, 
valleys, gorges, canons, cliffs, marine caverns, pot-holes, etc.; (2) 
the structures produced by moving ice, in glaciers and icebergs ; 
(3) the very subordinate structures produced by organic agencies 
especially by the boring and tunneling animals. 
ON THE TRACHYTE OF MARBLEHEAD NECK, 
MASSACHUSETTS. 
BY M. E. WADSWORTH, PH.D. 
Near Boden’s Point, on the north-west shore of Marblehead 
Neck, there is to be seen, exposed between high and low tide, the 
remains of a trachytic overflow. 
A short distance to the southward from this locality the granite, 
eruptive through the mica schist of the Neck, appears at its 
most northward point on this shore. This granite is here cut 
through and overlain by eruptive felsitic masses. This felsite 
shows the beautiful banding so characteristic of many rhyolitic 
rocks, especially when in lava flows, as well as often times when in 
dikes. At the same locality both the granite and felsite are cut 
by diabase dikes. 
