gS The American Geologist. Auf?ust, i898 
though precaution was taken not to reduce the grains fine 
enough to destroy their individuahty any more than was pos- 
sible. Portions were then digested on a water-bath, kept at a 
strong ebuHtion, in a covered dish, for several weeks with 
dilute hydrochloric acid, fresh quantities of which were added 
from time to time. A microscopic examination was made 
each time, after pouring off the extract, with the following 
results. — None of the mineral grains could be recognized as 
olivine after the digestion. This would indicate that the olivine 
had been dissolved and was the first mineral to yield to solu- 
tion, and therefore the most susceptible. A thin section of 
the rock showed that the olivine had been altered in an ap- 
preciable degree to serpentine. Of those minerals recognized 
under the microscope, the feldspar proved the least resistant. 
Particles of various sizes were noted, but generally only small 
fragments of the original grains were left, which evidently 
suggested that a part of the fragments of the feldspar had been 
entirely broken down by solution. Usually the largest grains 
were either partially or wholly covered with an opaque 
material, taken to be kaolin, which would probably account to 
some extent for their more refractory nature. While, on the 
whole, the augite grains were appreciably reduced and showed 
marked etching from solution, it proved the most refractory 
of the essential minerals in the rock. So far as could be seen 
the magnetite underwent but little change from solution. 
Apparently, the above results are in full accord with the 
mineral decomposition as shown under the natural conditions. 
That is to say, the decay, in- this special case, seemingly fol- 
lows, very closely, the order of solution of the various 
minerals. 
The extent to which investigation in this direction has 
been carried, probably shows the opposite order of mineral 
decay in rock weathering to be of a more frequent occurrence 
and therefore more general. 
The ferro-magnesian silicates, and especially those com- 
posed principally of the protoxides of the basic elements, such 
as iron and magnesia, are more susceptible to decomposition 
than the feldspars, owing to the readiness with which they 
are oxidized. Professor Merrill* states, that, "The evidence 
*See, "Rocks, Rock-Weathering and Soils," 1897, p. 234 
