214 AlfKUAL EEPORT SMITHSONIAN INSTITUTION, 1930 



As previously stated, there is strong evidence that the formation 

 of the maria occurred at a late stage in the period of vulcanism. The 

 earlier stages were characterized by the eruption of thousands of 

 craters, and these must inevitably have discharged large volumes of 

 gases. It is possible that these gases formed a temporary atmosphere 

 which, even if very tenuous, operated to check the rapidity of loss 

 of heat into space. Back radiation from gas molecules and fine par- 

 ticles of suspended volcanic dust would reduce the rate of heat loss. 

 Carbon dioxide is a common product of volcanic activity, and by 

 reason of its density it would be retained at the moon's surface for 

 a longer period than lighter gases. It predominates particularly 

 during the closing stages of vulcanism, and would thus accumulate 

 in largest quantities about the time of the extrusion of the maria. 

 Under cover of this protective mantle, the lavas which formed the 

 maria would assume a wider lateral extent than would have been 

 possible had they been extruded at the inception of vulcanism when 

 there was no atmospheric envelope to check loss of heat. Indeed, the 

 extrusion of the maria may only have become possible when an atmos- 

 phere was provided. If this gaseous envelope did exist, it would 

 not have remained long after the close of vulcanism, but would have 

 been dispersed into space, due to the moon's inability, owing to its 

 small mass, to retain gases against their diffusive tendency for any 

 extended period of time. 



The fissures on the high ground are those formed at the inception 

 of vulcanism. A different explanation must be offered for those 

 fissures seen upon the maria, especially as Shaler ^ states that these 

 phenomena were amongst the last of the moon's features to be 

 formed. These fissures appear to be rift valleys, and their occur- 

 rence on the maria where obvious signs of compression are visible, 

 is at first hard to explain. However, it is to be noted that the course 

 of the fissures is at high angles to the prevailing direction of the 

 ridges on the maria. In other words, the forces which raised the 

 ridges would not tend to prevent the formation of the fissures, or 

 to close them up if these latter were of earlier date than the ridges. 

 The rift valleys on the maria can be ascribed to faulting above fis- 

 sures in the surface upon which the maria were extruded. These 

 fissures ran parallel to the line of the compressive forces which 

 raised the ridges. In these deep-seated fissures the molten magma 

 was sinldng with the contracting substratum, and the surface of the 

 maria becoming relieved of support in these linear areas, sank, 

 forming the fault troughs now visible. 



An alternative suggestion to that made previously regarding the 

 origin of the ridges is that they were also formed above fissures in 



» Op. cit. 



