A PLEA FOR CiREATER COnPLETENESS IN CHEnICAL ROCK ANALYSIS.' 11s tV. I:
T
1irr.r.i XKXSI).
HE valuelessness to the mineralogist and geologist of man)-
of the analyses of niiiieA hubstances niade in earlier times is a fact too well-knowi to need substantiation. Defective methods of analysis, the difficulty of procuring pure reagents, and want of time for cshaustive examination have been largely responsible for this condition, hut lack of appreciation of the fact, now so well established. that substances present in small aniouiit niay have an important bearing on the discussion of results, has no doubt coiitributed in no siiiall measure to it. Whatever the causes, the result has been the necessity for a vast amount of repetition in aiialytical work, and it behooves the present generation of clieniists to heed \vel1 the warning and to work with a two-fold purpose i n view, that of lightening the enabling them to use our labors of those who come after us work with less supplenientary examiiiation, and of thereby enhancing our omii reputations 1)y nieriting encomiums 011 work that has stood the test of time. T h i s tiionition applies to the lithologist as well as to the chemist, and the former should seek to make or have his analyses made as complete as possible, and not, as is so often the case, be content with determinations of silica, alumina, tlie oxides of iron, lime, magnesia, the alkalies, aiid water; eve11 going so far sometimes as to ask the chemist to omit tests for other constituents that niay be present in sniall quantity for the sake of getting a greater number of more or less incomplete analyses acconlplishecl. Tlie latter, it is true, niay seIye tlie iniinediate purpose for which thcy were intended, h i t their inconipleteness may 011 the other iiand riot onlJ- conceal points fruitful of suggestion to the attentive mind, but, what is of still greater importance, they may be actually misleading. Enough instances of totally inaccurate conclusioiis to be drawn from tlieni have falleii under my own obsenation to fully justify this plea in favor of greater conipleteness in rock and mineral anal)-ses made for purely scientific purposes. b > r
1
Read at the Baltimore Meetiiig Uecriiiber
1". 1 ~ ; .
P L E A FOR G R E A T E R C O M P L E T E N E S S
IX
ROCK ANALYSIS.
91
This is shown by the difference between the following analyses, T h e specimens were taken and analyzed a t widely separated times and by different persons, it is true, but they were unquestionably from the same rock mass in which, however much the relative proportions of the different mineral constituents might vary within certain limits, there can be no reason to doubt the general distributioii of all the elements shown by the second analysis. Earlier Analysis
Later Analysis
sio,. 54.42 TiO,. A1,0, ................. 13.37 Cr,O,, Fe,Os. 0.611 FeO 3.52" MnO CaO 4.38 SrO BaO RlgO 6.37 K,O '0.73 Na.@ ................. 1.60 Li,O. trace H,O below 11ol'C. H,O above IIO'C. 2.76""
53.70 I .92 11.16 0.04 3.10.
................. ..................... .....................
................
.................. ..................... .................. ...................... ...................... ................. ..................
................ ......... ..... CO, .................. 1.82 P,O, ...................... so,. ...................... F ......................... c 1 ........................ .........
Less 0 for F1.
99.58
1.21'
0.04
3.46 0.19 0.62 6.44 11.16 I .67 trace 0.80
2.61 1-75 0.06
0.44 0.03 100.40
.I9 100.21
Another instance of a similar kind is given below. Here again certain differences are explainable by natural variations * From the fact that repeated determinations of the iron oxides in this and related rocks from the same region show always a great preponderance of ferric oxide, it is not iniprobable that the figures given for the two oxides in the first analysis were accidentally trausposed.
** In the published analysis it does not a p p e a r whether this is total water, or, as seems probable, only that reiiiaining above rw'C.
P L E A F O R G R E A T E R C O M P L E T E S E S S I S R O C K ANA1,YSIS.
93
i n the proportioiis of the constituent iniiierals, but it can hardly be doubted that TiO,, UaO, SrO, P,O,, aiid SO,. were present i n both speciiiieiis iii approximately the saiiie niiiouiits. In the earlier aiialpsis in this case deteriiiiiintioiis of soiiie supposed utiiiiiportaiit constituents were purposely oiiiitted o r oiily iiiatltqtialitatively. ivitli results which cannot be otlicrwisc than fatal to a full comprehension of the iiiineralogical iiature of the rock. i.'C 1
A:ial?
Ei,
SiO: . . . . . . . . . . . . . . . . . 44.3 TiO,. . . . . . . . . . . . . . . . . riot e s t .
. . . . . . . . . . . . . . . . ... , 2 , , . . . . . . . . . . . . . . . . . 4.64 . -., 1:t.o . . . . . . . . . . . . . . . . .
.11,0,
I
I:e,O
31rio
................
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Cat)
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