SKETCH OF GINA AND METHAWNA. 155 
Stavro district, where the later Spasmeno Vouno erupted mate- 
rial is more acid than the Stavro rock since, even making the 
alteration in the SiO, percentage mentioned further back, we 
find the acidity to be less than that of the former. The 
main Methana eruption also points the same way inasmuch 
as the earlier eruptions are all much more basic than the later 
flanking dacitic ones. One remarkable exception which has doubt- 
less also struck the reader must, however, be noted. At Kaimeni 
the site of the very last eruption in the whole region, we should 
expect to find, in accordance with the rule above, a quartz-bear- 
ing dacite,. perhaps like those of the mainland to the north; 
whereas we find a hornblende-hypersthene-andesite scarcely vary- 
ing in chemical composition from the much older eruption of 
Mt. Chelona,’ a gentle reminder that in the present state of our 
knowledge of the laws governing the chemical constitution of 
rocks generalizations must not be pushed too far.? While this 
exception is striking, yet it does not invalidate the rule enun- 
ciated above, and which we may regard as established for this. 
region, that the order of eruptions was one of increasing acidity. 
The relation of these rocks to one another in time is that most 
usually observed among the several possible cases.3 A closely 
parallel series of eruption products, though on a larger and more 
complete scale, has been described by Iddings* as occurring in 
the Yellowstone National Park. It is unfortunate that no data 
are at hand for determining the relative age of the rocks from 
any one group of the localities whose rocks are described by 
Kuch and Belowsky, since their comparison would be of great 
interest owing to the many points of similarity between their 
specimens and those described in the present paper. 
*It may be recalled that in the Kaimeni specimens are found a number of quartz 
grains which are conspicuously absent from the Chelona rocks. 
2As IDDINGS has pointed out (Orig. Ign. Rocks, Bull. Phil. Soc., Washington, 12, 
1892, p. 178) such reversions to an earlier type of differentiation products are not 
at all unusual. 
3Cf. ZIRKEL, op. cit., I., pp. 810 ff. 
4Erupt. Rocks of Electr. Peak and Sepul. Mt., 12th Ann. Rep. U. S. Geol. Surv., 
Washington, 1892, pp. 577-663. 
