1893.] ESSAYS. 113 



and south, while the whole of Messenger, Bancroft, and Newton Hills, 

 and the ridge on which the Polytechnic Institute is situated, are en- 

 tirely composed of glacial till or are parts of moraines. Union Hill, 

 the ridge on which Harvard and Woodland Streets are located, the 

 ridge west of Park Avenue at the north end and the hills east and 

 north of Lincoln Street are elevations partly of the mica schist rock, 

 but everywhere overlaid with morainic deposits or the overwash of this 

 material by later floods. The glacier in its passage over the valleys 

 left in each the older soil and covered it with the freshly ground rock- 

 meal, which makes up our boulder-clay. This is now seen best on 

 the northern and southern slopes of our higher hills, as on the south 

 side of Chandler Hill, the north side of Oak Hill, along the Blooming- 

 dale Road, and at the north end of Pakachoag. The examination 

 of glaciated rocks, and other outcrops and exposures in this vicinity, 

 reveals some interesting facts concerning the methods by which rocks 

 now are decomposing and breaking down into soils. These processes, 

 in part, must have also been pre-glacial. 



First, I hold here in my hand a piece of trap rock which comes 

 from the summit of Mt. Holyoke. You see how on the weather side 

 it has rotted, changing in color from dark gray to brown, and grow- 

 ing softer and more crumbly. The iron in the pyroxene has separated 

 from the quartz and become an oxide, rust. But if this fragment had 

 been a foot or two underneath the soil where vegetable tissue is decay- 

 ing it would have rotted four to ten times as rapidly. This other 

 pebble of the same kind has been thus buried and a part has flashed 

 oft'. This trap is found here and there in stone fences, and is called 

 ironstone. Here is another triangular fragment from Millstone Hill. 



This rock is eruptive granite, and full of minute crystals of pyrite. 

 Wherever this is exposed to the action of air and carbon dioxide dis- 

 solved in water, as adjacent to fissures, it is oxidized and the stone is 

 colored brown, becomes brittle and easily breaks. This is one of the 

 most common causes of the disintegration of rocks, all over the 

 world. 



Another is where rocks containing other iron bearing minerals are 

 decomposed through changes similar to those which break down trap 

 rocks. Hornblende and mica are such minerals, and the rocks of all 

 the mountains of the South are breaking down into the red soils seen 

 everywhere in Virginia, North Carolina and Georgia. These are 

 specimens. One is of tough elastic mica, another is from the Blue 

 Ridge near Asheville, N. C. It powders between the fingers like 

 chalk. A third, almost as soft, is a mineral called vermiculate, an 



