1885.] Pennsylvania before and after the Elevation, etc. 257 
Fic. 10,—Free young captured on surface of the water in glass jar containing speci- 
mens of the adult. Size 1™™, 
“ 11,—View of the head of a young Spirorbis older than the last with the begin- 
ning of the operculum and the right hand tentacle, seen from the dorsal side. 
PLATE XII. 
Fic. 12.—Side view of a larva of the same age. 
“« 13.—The same larva older, seen from the dorsal side. 
“. 14.—An older larva seen from the ventral side. 
“ 15.—Larval Spirorbis which has just begun to secrete its shell, shown with the 
collar and head partly protruded. 
“ 16.—An older larva with shell more completely formed than in the last. The 
head and collar are extruded. Size 2™™ 
A’ 
Ie. 
PENNSYLVANIA BEFORE AND AFTER THE ELEVA- 
TION OF THE APPALACHIAN MOUNTAINS, 
A STUDY IN DYNAMICAL GEOLOGY. 
BY PROFESSOR E. W. CLAYPOLE. 
HE geologist traveling or working among the contorted 
strata of Pennsylvania can scarcely escape being struck by 
the immense compression which the rocks of that part of the 
country experienced during the folding process which was the 
first stage in the formation of its mountain ranges. By this term 
I do not mean merely the condensation of the rock-masses by 
the tangential pressure to which the folds are due, but the actual 
shortening of the surface which must have resulted from the 
folding. 
Doubtless the thought has occurred to others, but I do not 
recollect seeing it put forward or developed to its legitimate con- 
clusions. Yet it is obvious that so extensive a corrugation of 
the earth’s crust manifesting itself by the production of several 
wide anticlinal arches, from which the present mountains have 
been carved, must have been accompanied by a diminution of the 
area over which those strata previously extended. 
To measure as nearly as practicable the extent of this contrac- 
tion of the surface and to set forth the more important conclu- 
sions deducible therefrom are the objects of this paper.’ 
To prevent undue extension in treating the subject, it will be 
necessary to assume certain propositions. These will be here 
1 An abstract of this paper was read before the British Association at Montreal in 
August, 1884. 
VOL, XIX,—NO. UI 74 
