PETROGRAPHY. 
II 
brand* and myself,f and which long experience has shown to be reliable. In many 
cases the rarer and less often looked-for constituents, as baryta and zirconia, were 
estimated, with results which are of considerable interest. In the statements of the 
analyses the molecular ratios are given in a column immediately following that of 
the percentage amounts. 
CLASSIFICATION. 
The classification adopted here is the quantitative one recently proposed, J a 
description of which it seems unnecessary to give. Comparatively little literature 
has as yet appeared expressed in the terms of the new system, and little in the way 
of describing or establishing types according to the suggestions made, so that the 
present paper may be regarded as illustrative of the practical application of the new 
classificatory principles and nomenclature to the actual study of rocks and the 
discussion of petrological problems. 
It is a somewhat difficult matter for the petrographer to change his conceptions 
of rocks so as to conform to the new requirements and to regard rocks primarily as 
magmas of which they are but the solidified forms, and more especially to keep 
the importance of the quantitative relations constantly in mind. This has proved 
true in my own case, and it has required considerable effort at times to keep to the 
narrow and untrodden path of a quantitative system based primarily on chemical 
composition, and to refrain from treading the broad road, easy from much use in 
the past, of qualitative systems based on mineral composition and texture. To 
what extent this effort to adapt the mental attitude to the new requirements is suc- 
cessful, it will be easier for the reader than for the writer to judge. 
Magmatic Divisions Represented, in the Roman Region. 
CLASS. 
ORDER. 
RANG. 
SUB RANG 
5. Canadare 
( i. Nordmarkase 
2 . Pulaskase 
3. Phlegrose, 
( 2. Vulsinose, 
I- 5- i. 3- 
I. 5. 2. 2. 
I Persalane . . . 
6 Russare 
( i. Miaskase 
I 3. Pulaskose, 
3. Beemerose, 
I. 5- 2. 3- 
I. 6. i. 3. 
7. Tasmanare . . . 
4 Austrare .... 
/ 2. Viezzenase. . . . 
i. Laugenase . . . . 
3 Tonalase 
3. Procenose, 
3. Appianose, 
3. Harzose, 
I. 6. 2. 3. 
I. 7. 1. 3. 
II. 4. *. *. 
II Dosalane 
\ 5. Germanare. . . . 
!2. Monzonase. . . . 
3 Andase 
( 2. Ciminose, 
I 3. Monzonose, 
( 2. Auruncose, 
II. 5. 2. 2. 
II. S . 2. 3 . 
II. 5- 3- 2- 
(6 Norgare 
2. Essexase 
I 3. Shoshonose, 
2. Vicose, 
II- 5-3-3- 
II. 6. 2. 2. 
7 Italare 
2. Vulturase. . . . 
2. Braccianose, 
II. 7. 2. 2. 
8 Campanare . . 
2. Vesuvase 
2. Vesuvose, 
II. 8. 2. 2. 
III. Salfemane . . 
( 7. Kamerunare . . 
( 2. Kamerunase . . 
( 3 Etindase 
2. Jugose, 
2. Fiasco nose, 
III. 7. 2. 2. 
III. 7. ?. 2. 
( 8. Bohemare .... 
2. Albanase 
2. Albanose, 
III. 8. 2. 2. 
* W. F. Hillebrand, Some Principles and Methods of Rock Analysis, Bull. U. S. Geol. Surv. No. 176, 1900 
t H. S. Washington, Manual of the Chemical Analysis of Rocks, New York (1904). 
t Cross, Iddings, Pirsson, and Washington, Quantitative Classification of Igneous Rocks (Chicago, 1903). 
Cf. Journal of Geology, X (1002), pp. 555-690. 
For the explanation of these symbols to indicate the magmatic position, see H. S. Washington, Prof. Paper 
U. S. Geol. Surv. No. 28, 1904, p. 13. 
