W. M. Fontaine— Vespertine Strata of Virginia. 37 



beyond it are still visible — arrange matters so that it is visible 

 to the eye looking through the tube, but not to the other, 

 directed at the small hole in the paper sheet This second sheet 

 will now appear to be traversed by a hole the same in size as 

 that cut through sheet No. 1. 



Cutting a small hole in sheet No. 2, matters are easily 

 arranged so that it appears within the hole which was before 

 Men within the tube. These experiments may be utilized in 

 showing the simultaneous accommodation of the two eyes. 



5. Tubes of this kind, blackened on the inside are very 

 convenient in studying color sensations. Using two sucli tubes, 

 look through one with the right eye, say, at red, through the 

 other with the left eye at green paper, illuminated by the direct 

 solar ray. The color sensations fade with marvelous quickness. 

 Transferring both eyes to either color, say red, the eye fatigued 

 by green sees the red greatly intensified, the effect being 

 rendered the more striking by the simultaneous impressions 

 received by the two eyes. Experiments in the combination of 

 color sensal - iggest themselves. 



The editor of the &:ie>t'tiri" Am-ricin has written sunietliimr 

 quite similar to some parts of this communication (Oct. 14). but 

 a comparison of it with my short note in Nature, Aug. 10th, 

 will show that I am not the borrower. 



Structure an<t ije^jnijil'i^i) distribution. 



The eastern border of the unbroken area occupied by the 

 Vespertine in the two Virginias may be taken as limited by 

 the Main Alleghany, in the northern and middle portions of 

 the area in question, and in the southern portion, by Peter's 

 and East River Mountains. 



As Prof. fin. B. Rogers has shown, the rocks of the Vesper* 

 tine, or No. X, compose the middle portion of the Main 

 Alleghany from the Potomac to Pocahontas county. This range 

 :ne distance south of the Potomac, is called 

 "the Eastern Front Ridge of the Alleghany," while from the 

 northern part of Pendleton county to the southeastern corner 

 of Greenbrier county, it bears the name "Alleghany." 



North of P - the range 



shows principally monoclinal westerly dips. The flexure- here 

 are comparatively broad and gentle. Hence the Vespertine 

 and associated strata first dip gently westward, and then pass 

 into a series of undulations, which extend nearly to the Ohio 



