18 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
gives m substance the views of Dr. Frazer, which have already been 
quoted. He refers to Dr. Frazer's " section 8" as representative of 
the South Mountain rocks and structure. In this section , as in the 
others in which the orthofelsite appears, notably section 9, the bedded 
orthofelsite is represented as overlying the quartzose conglomerate. 
Professor Frazer concludes his summary with the statement that "it 
is hard to avoid the inference that our South Mountain rocks repre- 
sent the Iluronian section of Murray and Logan." 
The geological map of Adams, Cumberland, and Franklin counties 
made by this final Pennsylvania survey refers all the rocks of the 
South Mountain region by the use of a single color to the "Azoic 
system." 
The Second Geological Survey of Pennsylvania, while recognizing 
the extent and crystalline character of the South Mountain rocks and 
emphasizing the absence of the Primal of Rogers, still failed to solve 
the problems of the structure of the region and the age and origin of 
its rocks. 
Professor Lesley has been very ready to acknowledge the incomplete- 
ness of the survey in the South Mountain, and since Mr. Walcott's 
determination of the age of its sedimentary rocks has appeared, he has 
expressed himself as desiring a new investigation of that region. 
While he claims that the survey of Pennsylvania "has been so minute 
and complete that comparatively little remains to be desired in the 
future," he adds: "The geology of the South Mountain is therefore 
[because of the new aspect given to it by the investigation of Mr. 
Walcott] in a very unsatisfactory condition and requires, in fact, a 
special and protracted investigation in the field. It is the most unsat- 
isfactory part of the work of the Geological Survey of the State." 1 
Subsequent workers in the South Mountain are indebted to the 
Survey for the superior topographical maps of the mountain made on 
a scale of 1 inch to 1,600 feet (about 3 inches to the mile) by A. E. 
Lehman. These maps are invaluable to the field geologist and render 
possible accurate areal mapping. Owing to a confusion of cleavage 
planes and bedding the dips recorded on these maps are not reliable. 
While the geological explorations of the South Mountain have been 
careful and minute and conducted by able geologists, the petrography 
of its rocks has never been thoroughly investigated. The microscope 
has not been used to assist in determining the nature and origin of the 
rocks, and to correct impressions colored by preconceived ideas or by 
an experience more or less limited to sedimentary structures. Under 
microscopic scrutiny and the comparative study of recent lavas, an 
increasing number of the so-called sedimentary rocks are proving to 
be igneous in origin. That such has proved to be the case in the South 
Mountain, while not surprising, is a fact of considerable significance, 
both because of characteristics which the rocks themselves possess and 
1 Report of the Board of Coram., pp. 5-6, 1893. 
