142 Correspondence. 



implies that the lunar depressions anciently called seas are really 

 deserted ocean-beds. Professor Phillips expressed himself in favour 

 of this doctrine at the late meeting of the British Association. It is, 

 I think, very desirable that the army of observers (including those 

 oificially instructed by the ''Lunar Committee") now besieging the 

 surface of our satellite, should direct particular attention to sea- 

 coast phenomena. Excepting where lines of craters have been con- 

 verted into an irregular succession of cliffs, the effects of marine 

 denudation can scarcely be mistaken. In the moon as well as in 

 the earth, they may be expected to consist of nearly straight lines of 

 escarj)ment apparently due to undermining and lateral encroach- 

 ment — winding lines of cliff apparently scooped out into bays with 

 headlands and semi-circular hollows — level areas under escarp- 

 ments, either terrace-shaped or graduating into the general level 

 of the neighbouring plain — ridges resembling sand-banks, running 

 across the plains or old sea-beds — isolated rocks, or groups of rocks 

 in the neighbourhood of escai-pments and especially of promontories, 

 etc. Among the apparent sea-coast phenomena which ought to be 

 minutely observed, I would venture to mention the following : — 



First, the more abrupt coasts of the Mare Imbrium. The great 

 Appenine escarpment is very instructive. It is exceedingly steep 

 and lofty, and has apparently been formed along the strike of the 

 metamorphic (?) rocks of which the mountain-range consists. It 

 has here and there been hollowed out into semicircular coves, which 

 have not reached the dimensions of bays. Along a considerable 

 part of its course, it overhangs a level beach terminated by a strik- 

 ing line of rocks resembling a natural break-water. Pakxs Nebu- 

 larum and Palus Putredinis are so wonderfully level as to point at 

 once to deposition parallel to a horizontal tluid surface. The Wedge- 

 shaped Valley of the Alps is a cleft (voe ?) nearly 12,000 feet deep. 

 Sinus Iridum is a semicircular bay " level almost as water," with 

 "abrupt and colossal cliffs" and promontories 140 miles apart. 



Second, the headlands, bays, and coves of Mare Serenitatis and 

 Mare Tranquilitatis, and the apparent beaches and sand-banks by 

 which their surfaces are varied. 



Third, the Mare Crisium, with its smoothly-rounded bays and 

 coves. The Promontorium Agarum resembles many terrestrial 

 headlands. Near the east edge there is a pass with island-like moun- 

 tains. In this deserted sea-bed I have frequently traced level 

 beaches extending from the bases of the cliffs a considerable distance 

 sea- ward, and then terminating suddenly. Similar beaches or level 

 plateaus may be traced in other lunar sea-beds. 



The so-called canals or rills, especially liyginus and Ariadeeus, 

 with their tributaries, tunnels, raised banks, etc., deserve to be 

 particularly examined with the view of discovering traces of their 

 being dried-up river channels.^ — Yours truly, 



D. Mackintosh. 



1 For information on the Selenography of the above and other regions of the moon, 

 see the Rev. T. W. "Webb's Celestial Objects j also articles by the same astronomer in 

 the Intellectual Observer. 



