PLATE X. 



Mare TranquilUatis and surroundings. Photographed by Ritchey, August 3, 1901, 2 

 hours SO 7ninutes a. m., central standard time. Exposure, tliree-fonrths second. 



Tlais plate includes nearly the whole of the INIare Tranquilitatis and, on the lower 

 margin, a portion of the M. Serenitatis. The large crater near the strait connecting 

 these niaria is Plinius. The highland nearest to it is the promontory of Acherusia. 

 On the southern, or upper, margin the view extends to the flanks of Theophilus. 



The most noteworthy features are the mountain ridges on the maria, the manner 

 in which the maria come in contact with the higher ground, the numerous crater 

 valleys, and the great "rills." 



It may be noted that ridges on the maria exhibit little trace of corresponding 

 troughs between them, such as are usually found in terrestrial mountain chains. 



The contact of the maria with the high ground has evidently resulted in the 

 partial melting of the walls of several vulcanoids. Where these structures are not 

 thus affected they are, apparently, in origin later than the formation of the maria. 

 The crater valleys are abundant on the right-hand or eastern side of the field. 

 Certain of them have been invaded by the lava of the mare. 



Some of the greater rills are very well shown. That on the extreme right side is 

 Hyginus. It will be observed that the course of these rills is at high angles to the 

 prevailing direction of the ridges on the mare. 



