Weathering of Diabase in Virginia. — Watson. 97 
sesqui-oxide of iron ; the magnesium carbonate being soluble 
in the carbonated waters would be carried off in solution. 
The table shows that 44.59 per cent of the fresh rock and 
62.44 per cent of the decomposed are soluble in dilute hydro- 
chloric acid and sodium carbonate solutions. Furthermore, 
that in passing from the fresh rock to the decomposed the total 
amount of watei has very greatly increased. 
The conditions, under which the degeneration of the 
rock here studied has taken place, indicate, that they have been 
essentially of the nature of chemical decomposition rather 
than of mechanical disintegration. Evidently, then we have 
a rock whose decomposition is practically complete accom- 
panied by a total loss of nearly two-thirds of the original 
material. While this loss, so far as data are available, is un- 
usually large, it is probably what would naturally follow under 
conditions where the process is approximately complete and 
the forces involved are mainly chemical rather than mechani- 
cal. Clearly, the increased loss in magnesia indicates that 
the olivines were the first of the rock constituents to yield to 
the agencies of decay, the feldspar and augite proving the 
most refractory. This order has been shown by microscopic 
study, which finds confirmation in the chemical analyses. 
With the above exceptions, the results and conclusions in 
general, closely agree with those reached by others in such 
cases. They point conclusively to the changes that have taken 
place in the decomposition of the rock being principally those 
of hydration and oxidation. 
Order of Decay in the Component Minerals. 
A microscopic study of the thin sections representing 
several successive stages in the rock decay, determined the 
least and most refractory of the minerals under the meteoric 
conditions for that locality. 
However, tests were made on several portions of the fresh 
and decayed material with reference to the order of the 
solubility of the minerals in dilute acid. Also, to ascertain if 
possible what relation, if any, existed between the weathering 
under the natural conditions and that of solution. 
The material was partially powdered and passed through 
a 40 mesh sieve, in order to secure grains of a uniform size, 
