Relation of Secular Decay of RocJiS. — Tan\ 27 
far down as thorough decay has extended. Below this the clay 
contains a greater percentage of soluble salts, is less finely divided 
and often contains undecayed masses of rock. Chert and pieces 
of quartz of moderate size are found in regions capable of sup- 
plying such substances, while in some of the soils derived from 
such igneous rocks as granite and basalt weathered and rounded 
boulders of disintegration, often fresh in the core, are common. 
This gradually passes into the quite thoroughly disintegrated Init 
not completely decayed rock, where all the minerals are present 
in original form, if not in substance, and where the original 
structure may be plainly traced, although the rock is as easily 
torn to pieces as so much unconsolidated clay. When this occurs 
in rocks of metamorphic origin, the planes of schistosity are often 
as well defined as in the original unaltered rock. This by de- 
grees grades into the perfectly fresh unaltered rock. 
(c) Dlstrihution of Residual Soils. 
Such soils as the above are found in various parts of the 
world. In northern Europe and northern America they are 
absent, having been swept from the surface by the continental 
ice sheets. In America, in the Hoosac mountains, in western 
Massachusetts, the rock was found, in the western end of the 
Hoosac tunnel, to be deeply disintegrated, though no residual 
soil was found.* 
At several points in southern Massachusetts, decay in the plu- 
tonic rocks is quite marked; and at Middlesex Fells, west of 
Cambridge, a diabase dike is deeply disintegrated, in places to a 
depth of twenty feet, to a gravel which can be easily worked 
with a shovel, t Rounded boulders with an undecomposed core 
are present in this mass. Similar cases occur in the very much 
rifted granite of Essex and Cape Ann, Mass., J and these cases 
are post glacial, the disintegration being the same in glacially 
transported boulders as in the bed rock. In the Middlesex Fells 
dike the rorln s moufonees surface of the decayed diabase still 
retain the glacial strife. These facts show how ra[)idly disinte- 
gration may proceed, even in a cold climate, in rocks of weak 
chemical composition. The process is not complete in these in- 
*Himt, Am. J. 8ci., 1888, xxvr, 190-213. 
IFirst noticed by Mr. J. B. Woodworth, of Harvard University. 
JShaler, Ninth Ann. Rept., U. S. Geol. Survey, p 567; Tarr, Am. J. Sci.. 
April, 1891. 
