100 Tlie America)) Geologist. Ausnst, isy> 
Loughridge* has shown this to be true in the case of soils, 
and further pointed out a Hmit as to time, with reference to 
the amount of sohible material removed by digestion. 
Turning now to the table, we find of the constituents de- 
termined, the alumina shows the greatest variation, with 
probably the lime next, although, in most cases, the iron in- 
dicates equally as great a variation. 
The magnesia remains fairly constant for the same rock 
in I artd II, while IV shows a wide variation, except when 
the maximum and minimum strength acids are com])ared. 
Undoubtedly, some importance can be attached to the 
presence of the large amount of olivine in the first two rocks, 
and the readiness with which this mineral is dissolved by 
hydrochloric acid. It seems fair to assume, therefore, that 
the uniformity in the magnesia determinations can be ac- 
counted for in part on this basis. This is apparently more 
fully warranted when we notice the variation in this consti- 
tuent to be equal to that of any other in rock IV, which is an 
olivine free rock. 
Each residue was carefully examined under the micro- 
scope, after filtering off the extract, and an abundance of un- 
decomposed silicate minerals was noticed in all, generally in 
a highly etched condition. With the exception of the strong- 
est acid used (i:i), the residues from the remaining acids in- 
dicated as great an abundance of magnetite as in an untreated 
portion of the same rock, which so far as could be detected 
showed apparently no etching. It follows then that the above 
acids are not sufficiently strong to dissolve this mineral, or 
else it is not a straight magnetite (Fea O^- A qualitative 
analysis, however, confirms that it is a pure magnetite, con- 
taining no titanium, magnesium, chromium or other elements 
sometiiiies found replacing a part of the iron. These results 
were further confirmed by powdering a crystal of pure Fes O'* 
and digesting for a short time on the water-bath with acids 
of various dilutions. It was found that as the strength of the 
acid was increased the solubility of the magnetite also in- 
creased, and when treated with concentrated hydrochloric 
acid only a few minutes were required to completely dissolve 
the mineral. While some of the mineral was dissolved, enough 
*Loughridge, R. H., Am. Journ. Sci., 1874, (3. S.) 7, 21. 
