276 BULLETIN OF THE 
The thin sections belonging to Professor Whitney’s collections were 
prepared by Mr. J. H. Huntington, late Assistant Geologist on the 
Geological Survey of New Hampshire, whose preparations have been far 
superior to the other American work on rock sections that I have seen, 
and are fully equal, if not superior, to the German work sent to this 
country. 
All classifications of rocks in vogue at the present time are, with one 
exception, confessedly artificial, as one can readily see by examining 
those proposed by Naumann, Blum, Von Cotta, Zirkel, Dana, Lang, 
Lasaulx, Rosenbusch, Roth, and scores of other writers of more or less 
note. The one exception is that scheme for the Tertiary volcanic rocks 
given by Baron Richthofen in the Memoirs of the California Academy 
of Sciences (Vol. I. Part IL, 1868), in which he announces that, be- 
ginning with the Tertiary age, volcanic rocks succeed one another in 
massive eruptions in the following order: propylite, andesite, trachyte, 
rhyolite, and basalt. Having laid down this order, his classification 
then becomes largely artificial. 
The usual classifications have been based on structure, age, minera- 
logical composition, chemical composition, and upon almost every part 
of the rock separately, but never on the entire rock as a unit. But 
little attention has been paid, when the scheme was based on min- 
eralogical characters, whether the minerals were foreign, original, or 
alteration products. If the same minerals existed in two rocks, no 
matter if the minerals in one were products of the crystallization of 
the cooling magma, and in the other alteration products, both rocks 
were classed as the same, even if there existed a difference of silica 
amounting to over twenty per cent. 
If we adopt the mineralogical classification, we are met by the fact 
that the feldspars cannot be distinguished from one another, in spite of 
the method of Des Cloizeaux, and its ingenious modification by Professor 
Pumpelly, — methods that fail exactly where they are most needod. 
Furthermore, our best chemists and mineralogists are not agreed as 
to what are the species of feldspar, —a point that it would be well to 
settle before the classification of rocks is made wholly dependent on the 
feldspathic constituent. 
Quartz, hornblende, augite, monoclinic and triclinic feldspars, and 
magnetic iron are found extending through rocks of ‘every age and of 
every percentage of silica, The quartz and hornblende exist as foreign, 
as indigenous, and as alteration products, — distinctions that should be 
noted in employing these minerals in classification. 
