8. E. Peal — Selenology. 501 



As regards its distance, motions, and topography, or mapping of 

 the surface details, our knowledge of the moon is well to the fore, 

 every little mound and fault being carefully recorded ; but what 

 the surface, so clearly seen, is composed of — whether of volcanic or 

 stratified rocks, or otherwise — no one can say. 



The structure and material of the surface is in fact a most hopeless 

 enigma, and this is all the more extraordinary when we recollect 

 that of all the heavenly host, our moon is the nearest body, and that 

 most frequently and easily seen, being totally unobscured by atmos- 

 phere or clouds, and easily observed with small telescopes and low 

 powers. 



To some extent this want of progress in selenology is due to the 

 old idea that the surface is now covered from pole to pole with the 

 remains of lava-lakes and stupendous volcanic explosions, a vast 

 cinder heap in fact, and is also partly due to the researches of Lord 

 Rosse, which (erroneously as we now know) were held to demon- 

 strate a maximum temperature of + 300° C. after 14 days' sunshine. 



But our want of progress is also undoubtedly due in part to the 

 general belief among astronomers, that a planet like our moon 

 could actually pass from the serai-incandescent and lava-crusted 

 stage, with huge vaporous envelope, direct to the cold, airless and 

 waterless stage, without an era of erosion intervening. 



Could retain, that is, its primeval volcanic surfacing throughout 

 the long era succeeding, while the temperature slowly declined, 

 and without the intervention of an era of erosion and deposition of 

 sedimentary rocks, as on our Earth. 



Judging by our own vast series of stratified rocks, we are led to 

 conclude that an exceedingly long temperate ' era of erosion ' must, 

 in the very nature of things, supervene on the heated lava-lake 

 stage, in all planetary development. 



It is over this part of the question that I invoke the aid of your 

 readers, i.e. to say, as geologists, whether they believe that a planet, 

 such as our moon, could retain from the igneous-molten ei'a, to its 

 present intensely cold, airless and waterless condition, its pristine 

 surfacing, the very poles themselves being covered (it is urged) 

 with large and small volcanoes, untouched by the hand of time. 



The experts who have specially studied the question, agree that 

 the maximum surface temperature under 14: days' solar heat, is at or 

 about zero, C. ; and the minimum during the lunar night falls to about 

 — 200° C. ; the mean being so low that solar heat raises no trace of 

 vapour, about the equator, at the limb. 



Thus the surface temperature of the moon must have fallen 

 enormously, and taken millions of years to do so. 



To aid your geological readers who may not be aware of the 

 peculiarities of the case I would point out that — 



1. The moon (unlike Mars) has no polar caps. 



2. There is an absence of all distinct colour in masses or in detail. 

 Over the entire surface we find warm greys and neutral tints 

 prevailing. 



3. There is a general and conspicuous absence of all evidence of 



