OcTOBEIt 2G, ISSo.) 



SCIENCE. 



501 



involves the further examination of the dependence 

 of a set of phenomena on tlieir distance from some 

 controlling condition, which can be called the direc- 

 trix, and which may change its position. This is 

 worthy of illustration. We find a simple case, in 

 which the directrix is motionless, in the escape of 





>4uille* .-oulccs. 

 «i 4es tobuUuoni d'Anotlidct. 



[ BaDca da T^nitBlIea, de NumakulitM, etc 

 L LajseUibraiiches » %-Altfcs UilUiitcs M lUic 



I T-AaeHibranaltua valrcsUiUaneca Mrt'/u. 



\ Banc* de TiimtLlles, de Nummutitc^, etc 



iCoqwillei litterales bieo conscrvccs oti non. 

 Vkci>, Q^niragoAKi ; dct»b dc v^tiaux & 

 puifajupon. 

 Niveau del UbuhlloDS d'ADntlldes. 

 I Dibtis de rn'inillM coulees, gres pcrfores, d 



ga<es during a volcanic eruption. The eruptive vent 

 is the directrix, and the various ga.'es are successively 

 given off from the lava when its temperature falls to 

 that below which they c.innot be occluded, the tem- 

 per.^tllre depending largely on the distance of flow 

 from the crater. An example in which the directrix 

 moves continually in one direction is seen in the 

 dependence of terrestrial day and night, with all 

 their attendant changes, from warmth to cold, ac- 

 tivity to rest, on the po>ition of the sun. One in 

 which the directrix moved fora lime in one direction 

 is seen in tlic relation of our drift-deposits to the 



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'retreating' margin of the continental ice. Far to 

 the norlhwanl of the margin, where the ice was 

 thickest and moved fastest, erosion was most active; 

 at a less distance, the ground-moraine was accumu- 

 lated at favorable points; at the margin, the Kame 

 gravels were deposited ; and farther south, the brick- 

 clays settled where they found quiet water: hence all 



these m.ay be chronologically equivalent in passing 

 from south to north, although at a given point we 

 sliDulil find a vertical sequence from scratched rock, 

 through ground-moraine and Kame gravels to brick- 

 cl.iy. An effect of irregular motion of the directrix 

 will be seen in the shifting of all those physical and 

 chemical actions going on within the earth, and 

 dependent for their proper temperatures and press- 

 ures on their depth below the surface; for this 

 depth, or the distance from their directrix, is con- 

 tinually, though very slowly and irregularly, chan- 

 ging, — decreasing, while the superincumbent 

 mass culminates in a laiul-surface that is losing 

 ground by erosion; increasing, while it is receiv- 

 ing new material below tlie sea. A regular oscil- 

 lation of the directrix is presented in the swinging 

 of the sun north ami south of the equator, carry- 

 ing the seasons, the wind-systems, and the length 

 of the day, in its train. Finally, the case in point 

 shows us an irregular shifting of llio shore-line 

 directrix as the land slowly rises ami falls. As a 

 ; first result of the dependence of deposits on their 

 distance from the shore-line, we shall find that 

 those formations which are at any given moment 

 contemporaneous, or horizontally eqtiivalent, are 

 the very ones already seen at any given point in 

 vertical sequence. Secondly, when we view a broad 

 set of deposits accumulated during a shifting of the 

 shore-litie, it will be seen, that while the band of con- 

 glomerate or sandstone is continuous for considerable 

 distances, and apparently of contemporaneous forma- 

 tion throughout, it is not so in reality ; for the lines of 

 composition are not lines of deposition, and one part 

 of the conglomerate is distinctly of later date than 

 another, and really contemporaneous with the clay 

 overlying tlie latter. This is illustrated in fig, 2, 

 and shows the complete abandonment of the old 

 itleas concerning universal formations. Instead of 

 supposing that contemporaneous deposits are of uni- 

 form composition throughout, we must now admit 

 th.at tliey necessarily vary. 



M. Kutot's paper was prepared especially for the 

 explanation of Belgian geology. Before it could 

 serve as a guide to the meaning of our broad paleo- 

 zoic strata, there should be added a consideration of 

 the geographic conditions of limestone-making, and 

 of the former greater strength of transporting agen- 

 cies required by our old conglomerates. It would 

 have been well to consider Phillips's suggestion con- 

 cerning continuous subsidence at irregular rates, in 

 which the shallowing is produced by deposition 

 instead of by elevation; for, although this is quite 

 inadequate to explain the changes in the hejivy Ap- 

 palachian sediments, where shallow-water sandstones 

 sometimes quickly follow deep-water limestones or 

 shales, it may serve in certain cases of smaller meas- 

 ure, which M. Rutot has interpreted as the effects of 

 oscillations. On the other hand, the occurrence of 

 elevation after and in spite of deposition might be 

 emphasized to show the rather one-sided aspect of tlie 

 conclusiims lately discussed by the English geologists, 

 who too often consider erosion and deposition as 

 almost the chief causes of change of level. 



