:>v. 



PROCEEni\GS OF THE POLYTKi'JfXIC ASSOCIATWX. 819 



parts of a ring inouutain, which projects fruiii the lowlands by Its 

 side. 



The dark portions of the moon's surface, formerly supposed to be 

 sea-^, approach Tycho, upon this side, much tlie nearest, as if the 

 country in every otlier direction iiad been submerged by the mate- 

 rial of which the rays themselves are composed. The banks of the 

 whasm retain their distinctive character as rays much closer up to the 

 parent basin than any other ray which proceeds from it, leaving 

 the inference tliat the How Avas intercepted at an early period in the 

 history of Tycho and its surroundings, which would account for the 

 ancient formations on that side liaving escaped submergence. 



This great double ray partakes of the common elevation in the 

 neighborhood of Tycho, wliile its altitude decrease*! as its distance 

 from the center increases, until its distinctive character is finally lost 

 in the low-lands, showing that the system of Tycho, like all the 

 other ray systems of the moon, is highest at its connection with the 

 central basin from which the flow has taken place. But this is so 

 universally accepted as being the case, that I notice it here merely to 

 call attention to it, as it points with ]>eculiar significam-e to the 

 causes which produced those structures. 



Evidence of Sjidimentary JDiaFrs. 

 It often happens, that when a ray encounters the rampart of a ring 

 mountain, that it finds its way into the basin through the fissures 

 and fractures between the craggy masses of whi(*h the ring is more 

 or less composed, in which case the debris swept from between those 

 masses may be found in the form of drift under the inner wall on the 

 side next that from which the ray ))roceeded. This appearance of 

 drift frequently spreads itself over the whole floor of the basin, 

 leaving it marked with streaks of difterent degrees of light and 

 shade; such a ray usually continues across the basin, passing out at 

 the opposite side from which it entered, showing that whatever the 

 active agent might ha\t' 1»een from which the ray was deposited, 

 that it was of a nature which admitted of its readily ol)eyiiig the 

 direction of the impulse impressed upon it. Thitt it was not lava, 

 is demonstrated by the fict that the basins across which those rays 

 pass are never filled up, l;ut merely marked to a greater or less 

 degree with forms suggestive only of sedimentary deposit on which 

 the shadows of the rampart are still cast by the rising or setting 

 sua, showimc that theii' surfaces are much l)elow the river, and as 



