BASCOM.] 
AGE OF THE ROCKS. 29 
COMPARATIVE AGE OF ACID AND BASIC ERUPTIYES. 
FIELD RELATIONS. 
The relative age of the acid and basic volcanics is an interesting 
question, a question to which it is not possible with our present knowl- 
edge to give an entirely satisfactory answer. 
On the one hand we find the acid rocks occupying, as a rule, the 
lower altitudes, and even overlain by the basic rock. Occurrences of 
the latter nature are located as follows: 
(1) At the junction of Gladhill's road and the Fountaindale turn- 
pike, in the northeast angle of the roads, a slight excavation exposes 
an outcrop of gray, slaty felsite overlain by greenstone. 
(2) Not far from the source of Minie Branch there is exposed by the 
cutting of the stream a gray felsite, overlain by greenstone. 
(3) On the road from Fountaindale to Fairfield, southeast of Jacks 
Mountain, a much-altered felsite is overlain by greenstone. 
(4) Lastly, on the Fountaindale turnpike, about half a mile above the 
Emmetsburg tollgate, the same superposition is exposed by the roadside. 
The only published statement of opinion as to the relative age of the 
volcanics has been made by Mr. Keith. 1 From his observations of the 
relative positions of the acid and basic rocks in Maryland, Mr. Keith 
expresses himself as confident that the " quartz-porphyry underlies the 
diabase." 
On the other hand, over against this somewhat meager proof must 
be considered a few facts of another sort. Professor Williams reports 
that a position of the acid and basic rocks the reverse of that which has 
been described in the Monterey district occurs north of that district. 
Throughout the northern portion of the range the acid eruptives, which 
abound almost to the exclusion of the basic eruptives, form the moun- 
tains, while the latter occupy the valleys. 
We must, moreover, bear in mind that the principles of stratigraphy 
employed in determining the age of sedimentary rocks may prove very 
misleading if applied to igneous rocks. The younger lava may be 
found overlain by the materials of an earlier eruption where the 
former, in coming to the surface, breaks through the older formation. 
The latest eruption may fill the depressions. 
Where two different lavas occur on the same level, as is the case 
south of the Clermont House, the lava (in this case the acid lava) occur- 
ring as a narrow strip inclosed by the other might be an intrusive 
eruptive, and thus younger than the inclosing basic lava. 
PROBABLE EXPLANATION. 
With these principles in mind, with conflicting field evidence, and in 
the absence of genuine dikes of either acid or basic character, the data 
for the determination of the comparative age of the acid and basic 
i Loc. cit., p. 367. 
