bascom.] CHEMICAL COMPOSITION OF THE ACID ERUPTIVES. 
61 
Messrs. Hague and Iddings l make the statement " that the degree of 
crystallization developed in igneous rocks is mainly dependent upon 
the conditions of heat and pressure under which the mass has cooled, 
and is independent of geological time." 
On the question of devitrification the writer finds no more direct 
expression of opinion, but the fact of devitrification is recognized by 
Iddings, Williams, Cross, and Diller. In none of the felsites elsewhere 
described have the varied structures of the modern rhyolites been more 
perfectly and conspicuously preserved than in the aporhyolites of the 
Eolith Mountain. 
CHEMICAL COMPOSITION OF THE ACID ERUPTIVES. 
The chemical study ot the South Mountain rocks has been rendered 
easy by a number of analyses of these rocks made by the late Dr. Genth, 
under the auspices of the Second Geological Survey of Pennsylvania. 
Analyses of the sedimentary, acid, and basic igneous rocks of South 
Mountain are distributed through pages 252 to 282 of Eeport COC. 
The analyses of the acid volcanics have here been brought together 
and tabulated. 
Table of analyses. 
I. 
II. 
III. 
70.76 
.11 
12.64 
2.63 
IV. 
V. 
VI. 
Si0 2 
79. 970 
79. 920 
.190 
12.400 
2. 950 
76.62 
75. 570 
.190 
11.940 
3.830 
75. 31 
.05 
12.96 
3.05 
P 2 O fl 
A1,0 3 
13. 860 
12.26 
2.08 
4.03 
Fe 2 Oi 
FeO 
2.700 
Ti0 2 
.68 
.41 
MgO 
1.230 
.220 
.26 
.34 
2.57 
CaO 
Trace. 
6.490 
2.320 
.11 
6.15 
K 2 
5.250 
3. 010 
3.15 
5.11 
Na.O .. 
H 2 
1.53 
.440 
.40 
.270 
.61 
Total 
95. 038 
100.71 
100. 77 
99. 09 
100. 06 
100. 65 
I. Fissile green schist. Between Pitie Grove Furnace ami Laurel forge. 2 
II. " Orthofelsite." "One-fourth of a mile north of Lerew's store." 3 
III. "Orthofelsite." ''Cut on turnpike 5 miles northwest of Petersburg, Cumber- 
land County." 3 
IV. "Laminated felsite." "East of Bigham Copper Mine." 4 
V. "Laminated orthofelsite. 7 ' "One-fourth of a mile southeast of Caledonia Fur- 
VI. 'Finely laminated orthofelsite." "One-fourth of a mile west of Cole's saw- 
mill, on the Shippenburgroad." > 
Their general uniformity and their close agreement with the analyses 
of typical rhyolitic lavas is the most striking feature of these analy- 
1 On the development of crystallization in tlio igneous rocks of Washoe, Nov., with notes on the 
geology of the district: Bull. U. S. Geol. Survey No. 17, 1885, p. 40. 
2 Analysis made by A. S. McCreath for commercial purposes ; alkalies undetermined. 
3 Analyses made by Dr. Genth and Henry Trimble, Second Geol. Surv. Pa., Vol. CCC, pp. 263-269. 
4 Analysis made by C. Han ford Henderson, of Philadelphia, The copper deposits of the South 
Mountain; Trans. Am. Inst. Mm. Eng., Vol. XII, 1884, p. 90. 
