Geology. 315 



are classed under a number of heads, certain wrinkles upon the 

 surfaces of the maria appearing to correspond to the usual earth 

 type of mountains due to crustal shortening ; the other types 

 appearing to have had an igneous origin and to be formed of 

 viscous lavas which have solidified with very steep slopes. 

 Clearly marked faults are rare upon the moon, though cracks, as 

 indicated by the so-called "rills," are abundant. 



The almost complete absence of evidence of shrinkage and con- 

 sequent tangential or mountain-making thrust is one of the 

 greatest fundamental distinctions between the history of the 

 earth and its satellite and for a better understanding of which 

 we may have to await a fuller knowledge of the causes of terres- 

 trial contraction. 



The ray systems, whitish streaks which radiate from certain of 

 the prominent craters to distances of several hundred miles, 

 crossing plains and vulcanoids alike, are among the most enig- 

 matical features of the moon's surface, becoming strongly visible 

 only about two days after sunrise and disappearing at a some- 

 what lesser interval before sunset. The author concludes that 

 they are probably fumarolic deposits within and upon the crust 

 from extremely deep-seated fissures and that they are of ancient 

 origin, a view which precludes their consisting of ice, since in 

 that case a gradual evaporation and dispersal of the water vapor 

 would take place at even the lowest temperatures. The}'' are 

 regarded as most probably due to some crystalline and fluorescent 

 material which does not reflect the sunlight until a considerable 

 elevation has been attained. 



Considering that these ray systems are of considerable geologi- 

 cal antiquity, the author raises the question as to why in a sphere 

 free from erosion they have not become veiled by meteoric dust. 

 Of a number of suggestions presented to explain this feature the 

 most reasonable are that either the amount and importance of 

 meteoric dust received by the earth and moon have been greatly 

 overestimated, or else that we have misjudged the age of the 

 moon's surface and the ray systems are not of great antiquity. 



As to the permanence of the lunar relief, it is evident from an 

 examination of the plates "that there is some agent which has 

 operated to break down the more ancient topographical features. 

 There is an evident difference of aspect between the walls of the 

 older vulcanoids and those of newer formation." In fact all 

 stages of obliteration may be traced to large nameless vulcanoids 

 whose ruins only a careful examination will reveal. As water 

 has evidently never acted, the most probable cause is assigned to 

 be alternate expansions and contractions of the superficial crust 

 during the lunar day and night. Whatever the agent of decay 

 may be, the numberless superpositions of vulcanoids and the 

 extremely ruined character of the most ancient is indicative of a 

 long and complex volcanic history. Professor Shaler as the 

 result of his studies disbelieves in any atmosphere or present 

 volcanic activities even of solfataric stages, but in view of the 



