GEOLOGICAL CLIMATE. 279 



their diameter. These tubes have few or no discs 

 upon them but are covered with spiral striae, giving 

 the appearance of each tube being formed of a 

 twisted band. The above characters prevail in all 

 parts of the wood, but are slightly modified in 

 different rings. Thus the outer zone is broader in 

 some than in others, the disc bearing fibres of the 

 outer zone are sometimes faintly marked with 

 spiral striae ; and the spirally marked fibres of the 

 inner zone sometimes bear discs. These appear- 

 ances suggest the annual recurrence of some 

 special cause that shall thus modify the first and 

 last formed fibres of each year's deposit, so that 

 that first formed may differ in amount as well as 

 in kind from that last formed ; and the peculiar 

 conditions of an arctic climate appear to afford an 

 adequate solution. The inner or first-formed zone, 

 must be regarded as imperfectly developed, being 

 deposited at a season when the functions of the 

 plant are very intermittently exercised, and when 

 a few short hours of sunshine are daily succeeded 

 by many of extreme cold. As the season advances 

 the sun's heat and light are continuous during the 



