1874.] On (he Comparative Value of certain Geological Ages. 145 



Finally, it may be mentioned that these nodules also grow in circum- 

 ference, by the alveolar septa of the neighbouring alveoli gradually be- 

 coming thickened, while, at the same time, the epithelium of the corre- 

 sponding alveoli undergoes the changes before described. The capillary 

 vessels of these parts show the same changes as were mentioned in the case 

 of the guineapig's lung being transformed gradually into nucleated fibres, 

 which must be supposed to be, for a certain time, still permeable by 

 coloured fluids. 



If we summarize the results thus described, it is evident that the 

 changes in the process of miliary tuberculosis in man are only to a limited 

 extent similar to those which occurr in the process of artificial tu- 

 berculosis in guineapigs. In the lung of tuberculized guineapigs the 

 first structural changes are characterized, briefly speaking, by the appear- 

 ance of perivascular lymphangial nodules, whereas the changes of the in- 

 teralveolar tissue and the alveolar epithelium form only a secondary pro- 

 cess. In miliary tuberculosis of man, on the other hand, we see that the 

 first changes take place in the alveoli and interalveolar septa, and these 

 changes are followed by the appearance of perivascular cords. 



It is therefore probable that, in artificial tuberculosis of the lung of the 

 guineapig, the parts first attacked are the small branches of the pulmo- 

 nary artery or pulmonary vein, whereas in the miliary tuberculosis of man 

 the capillary blood-vessels of the alveoli seem to be the tissue from which 

 the action of the morbid agent starts. 



II. " On the Comparative Value of certain Geological Ages (or 

 groups of formations) considered as items of Geological Time." 

 By A. C. RAMSAY, LL.D., V.P.R.S. Received December 16, 



1873. 



(Abstract.) 



The author first reviews briefly several methods by which attempts 

 have been made to estimate the value of minor portions of geological 

 time, such as : calculations intended to estimate the age of deltas, 

 founded on the annual rate of accumulation of sediments ; the astro- 

 nomical method followed by Mr. Croll, in connexion with the recurrence 

 of glacial epochs ; the relative thicknesses of different formations ; and 

 the relation of strong unconformity between two sets of formations in 

 connexion with marked disappearance of old genera and species, and the 

 appearance of newer forms. Having shown that none of these methods 

 give any clear help in the absolute measurement of time in years or 

 cycles of years, even when founded on well-established facts, he proceeds 

 to attempt to estimate the comparative value of long portions of geo- 

 logical time, all of which are represented by important series of 

 formations. 



