PBKSIDBNT's address — SECTION c. 77 



contraction is still progressing rapidly in zone (b), the latter 

 tends to shrink aAvay from zone (a) so as to leave it unsup- 

 ported. Zone (a) consequently is thrown into wrinkles to 

 accommodate itself to the surface of the zone of maximum 

 contraction (b), the line of junction between (a) and (6) being 

 termed the zone of no strain. 



Volcanic outbursts may take place in this third stage from 

 tlie same causes as in the second stage, the chief difference 

 being that the material from the nucleus has now to pass 

 through three zones instead of the two before it reaches 

 the earth's surface. 



If this theory be applicable to the original crust of the 

 earth it should apply equally to explain the causes of vol- 

 canic action in the present crust, the chief alteration in the 

 conditions being that the zone (a) of minimum radiation and 

 maximum is now largely composed of sedimentary rocks, 

 instead of being wholly volcanic, as they probably were when 

 the original crust Avas formed. 



The theory above quoted is only one of many which may 

 be only partially true. It serves, however, to show that 

 secular contraction of the zone (Z»), and perhaps expansion 

 of the solidifying zone (c), are probably the primary causes of 

 volcanic action. 



Before explaining how heavy sedimentation may assist 

 secular contraction in producing volcanic eruptions, reference 

 must be made to the probable physical condition and chemical 

 composition of the rocks composing these zones. If the 

 assumption be correct that the rate of increase in temperature 

 downwards in the earth's crust after the line of mean surface 

 temperature is passed is about 1° Fahr. for every 63 feet of 

 descent, it follows that at a depth of about six or seven miles 

 a temperature of about 700° Fahr. might be reached, wliich 

 is probably, for many well known reasons, the lowest 

 temperature at which granite is likely to form, so that this 

 depth of seven miles may mark the extreme upward hmit of 

 the granites, which, for convenience of reference, may be 

 termed the isogeotherni of 700° Fahr. This depth, there- 

 fore, should theoretically lie below the zone (a) of maximum 

 compression ; and earthquake observations are somewhat in 

 harmony with this theory, as, except in the neighbourhood of 

 volcanoes, it is unusual for the point of origin of an earth- 

 quake to lie at a less depth than from five to six miles, and 

 earthquake shocks are more liable to originate in the zone (b) 

 of maximum contraction than in the zone (a) of maximum 



