PLATE XXII. 



MARE TRANQUILITATIS AND SURROUNDINGS. PHOTOGRAPHED BY RITCHEY, AUGUST 



3, IQOI, 2 HOURS 30 MINUTES A.M., CENTRAL STANDARD TIME. EXPOSURE, 



f SECOND. SCALE, THREE-FOURTHS METER TO MOON*S DIAMETER. 



This plate includes nearly the whole. of the Mare Tranquilitatis and, on the lower margin, 

 a portion of the M. Serenitatis. The large crater near the strait connecting these maria 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 in this plate 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. Some 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 

 (see p. 44). It will be observed that the course of these rills is at high angles to the prevailing 

 direction of the ridges on the mare. 



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