May 4, 1883.] 



SCIENCE. 



357 



the making of ' coves,' as may be perceived 

 b}' the following considerations. The backward- 

 cutting of a lateral stream can form a water-gap 

 only where the longitudinal valley into which 

 the lateral stream flows is decidedly lower than 

 the longitudinal \-alley on the other side of the 

 dividing-ridge ; for, if there is no such differ- 

 ence of level, the pass through the ridge be- 

 tween the two will be eroded more and more 

 slowh' as it is lowered, and finally it will remain 

 at practically a constant altitude above the 

 valleys on either side. It can never form a 

 drainage channel joining them : but, if the 

 longitudinal valleys are of different heights, 

 the result as described by Low! may be pro- 

 duced ; or, if a broad plateau-fold is bordered 

 by a deep vallej', its lateral streams may finallj- 

 head up in coves or circular valleys, like those 

 south of the west branch of the Susquehanna, 

 and at manj' other points in the Appalachians. 

 The geological map of Penns^ylvania (18.58) 

 shows these admirably. 



Now it may be asked. Are not the upper 

 vallej's of the Susquehanna, and of the other 

 rivers that break through the Blue mountain, 

 merelj' large examples of ' coves ' ? There are 

 two objections to this explanation. First, 

 what became of the head waters of these riv- 

 ers before thej' had a south-easterlj- outlet ? It 

 seems most probable, that the many pre-exist- 

 ent streams in each river-basin concentrated 

 their waters in a single channel of overflow, 

 and that this one channel survives, — a fine 

 example of natural selection. Second, how 

 does it happen — notably in the case of the 

 Susquehanna just above Harrisbiirg — that 

 several deep water-gaps have been formed one 

 behind the other ? Such an arrangement might 

 naturall}' result if the valle3' were antecedent ; 

 but it is difficult to account for if the several 

 gaps result from the backward erosion of acci- 

 dental lateral streams. 



Lowl thinks that faults are greater obstacles 

 to rivers than folds. lie says, that even if 

 river erosion could, under certain favorable con- 

 ditions, keep pace with mountain folding, it 

 does not follow that it could control a fault : 

 for that would implj' that the fault was formed 

 gradually, and that its throw increased at a 

 constant though imperceptible rate ; and this 

 he considers entirely unwarranted (408). It 

 is certainljr a difficult matter to understand the 

 mechanics of such faults ; and yet our ideas 

 concerning them must conform to the facts as 

 the}' occur in nature. In spite, therefore, of 

 a natural preference for an active growth of 

 faults, we are compelled, when we see streams 

 running acros? them from the downthrow to the 



upthrow side, to accord them a slow growth. 

 Tennessee shows many examples of this para- 

 doxical nature ; and some of the faults thus dis- 

 regarded, or, we might say, corrected, have a 

 throw of several thousand feet. On the other 

 hand, it cannot be denied that manj' faults have 

 had a controlling influence on stream-courses ; 

 and we must therefore admit here, as above, 

 the possibilitj' of vallej'-cutting being stronger 

 or weaker than orographic movements. The 

 variet}' that is to be seen in the phj-sical fea- 

 tures of the earth, and that is consequentl}' to 

 be looked for in the conditions which deter- 

 mined thein, is so great that it demands almost 

 equal variety in the theories for their explana- 

 tion. W. M. Davis. 



Cambridge, Jan. 12, 18S3. 



THE ORIGIN, AFTER BIRTH, OF ASPI- 

 RATION OF THE THORAX. 



The negative pressure in tlie pleural cavities, which 

 plays such an important part in the resph'atory mech- 

 anism of the adult mammal, and also exerts a marked 

 influence upon tlie flow of blood and lymph, is known 

 not to exist in unborn or stillborn mammals which 

 have never breathed. It has hitherto been assumed, 

 however, that it was established with the first in- 

 spiration ; and all theories as to its mode of produc- 

 tion have been controlled by this belief. Hermann, in 

 an interesting paper [Pfldg. archiv, xxx. 276), shows 

 that this assumption is incorrect. In infants which 

 have lived and breathed after birth for peiiods of from 

 one hour to eight days, there is found on experi- 

 ment to be no negative pressure in the pleural cavity 

 when the chest-walls are in their death position. 

 This fact leads, necessarily, to important results in 

 regard to the respiration of young mammals. Their 

 lungs in expiration can contain hardly any of what 

 in the adult is known as 'residual' air: they con- 

 tain still some air imprisoned in the air-cells, and 

 causing them to float in water. But this ' minimal ' 

 air is practically no more than what remains in a 

 piece of adult lung squeezed between thumb and fin- 

 ger. Except this minute quantity, there is in the 

 new-born mammal no stationary air. At each in- 

 spiration, air direct from outside enters the alveoli 

 of the lungs; and, at each expiration, air from the 

 alveoli is expelled, leaving the lungs practically empty. 

 Hence the renewal of the air in the lungs is much 

 more eflicient than in adults. The high percentage 

 of oxygen which the alveolar air must contain is 

 probaijly correlated with the more active oxidations 

 known to occur in the young animal. The question 

 naturally arises, What is the object of the residual 

 air in the lungs, and the negative pressure in the 

 thorax, which we find established later? Hermann 

 suggests three possible objects: 1°. Aspiration on the 

 veins, promoting bloodflow to the heart; 2°. More 

 uniform composition of air in the alveoli [and, we 

 may add, more uniform temperature] ; 3°. The pres- 

 ence of a certain store of air in the lungs in the case 

 of a temporary stoppage of the breathing-movements. 

 It remains to be seen at what age and rate the nega- 

 tive pressure in the thorax is developed. It is obvi- 

 ously brought about by a more rapid growth of the 

 thorax than of the lungs. H. Newell Martin. 



