193 



[Lesley. 



JVtites Oil thi: Modeln Exhibited at the Meeting Noooiiber 1'.), 1880. By 



J. P. Lesley. 



These models were made by and under the superintendenee of Mr. Ed. 

 B. Harden, Topographical Assistant of the Second Geolosjical Survey of 

 Pennsylvania, and form part of a collection of models in the Museum of 

 the Survey, No. 907 Walnut street, Philadelphia. 



Each is constructed on one and the same vertical and horizontal scale to 

 avoid structural distortion ; but the scale of the first is 1 mile : 1" ; that of 

 the second 1600': 1" ; and that of the third 800' : V. 



The model of the Seven Mountains represents a district of anticlinals 

 and synclinals, forty miles long, extending along the north side of the Kish- 

 icoquillas valley in Centre and Union counties. Parallel anticlinal ranges 

 of Medina and Oneida, dying down eastward into Union, and westward 

 into Huntingdon counties, form a mountain belt between the Siluro-Cam- 

 brian lowlands of Kishicoquillas valley in Mifflin and Snyder counties in the 

 south, and Pine Creek, Brush and Nittany valleys of Clinton and Centre 

 counties on the north. The broad rounded spurs sinking with the axis 

 of each anticlinal beneath the Clinton red shale and fossil iron ore beds, 

 are finely shown. In the heart of the region the anticlinal mountains split 

 open, and show long narrow deep vales of Hudson river slate, while the 

 synclinals contain long narrow strips of Clinton red shale. 



In two places occur diagonal upthrow faults on a large scale, one through 

 the Stone Mountain at Greenwood furnace ; the other (in a prolongation of 

 the line of the first fault), some miles further to the north-east. 



Both faults have the western portion thrown northward and backward 

 towards the north-east, as in a diagonally splintered arm bone drawn to- 

 gether by the contraction of the muscles ; and this structure is plainly ex- 

 hibited by the termination against each side of each fault of the Medina 

 mountain crest, and of the Oneida terrace which always accompanies it. 



The model of the Stone Mountain fault shows the structure on a large 

 scale, and especially the slight curves at the ends of the hypothetical 

 straight line of the fault, as well as the crushed and packed-in condition of 

 the red shales around the north-east end of the fault. In the case of the other 

 fault, there is a much greater complication ; for two cross faults, at the two 

 ends of the main slide line, parallel to each other, must be imagined to 

 satisfj^ all the surface conditions. 



The model of the Mammoth bed floor in the Schuylkill county is the first 

 of this kind constructed in the Anthracite region. There is a model of the 

 same kind in the Museum of the Towne School of the University of Penn- 

 sylvania, showing the floor of the Pittsburgh coal bed in Somerset county, 

 Pennsylvania, which I had made by my students from data obtained by a 

 special survey ordered by the Board of Commissioners of the Geological 

 Surve}-- in 1875. But the Anthracite basins are deep, steep sided, and with 

 their separating anticlinals sometimes overthrown towards the north and 

 collapsed. These are the most sjtriking features of this model. 



It also exhibits for the first time another unexpected and very important 



PROC. AMER. PHtLOS. SOC. XIX. 107. Y. PRINTED JANUARY 18, 1881. 



