Febbxjabt 1, 1907] 



SCIENCE 



165 



nologically. The phrase, 'Historical Geol- 

 ogy/ which forms the title of the largest 

 section of Dana's 'Manual,' involves a dis- 

 tinct clarification of the general view of 

 the science. Starting with this conception, 

 he, of course, dealt with the earliest forma- 

 tions first. In the treatment of each era 

 he endeavored to reconstruct, from the evi- 

 dence afforded by the kinds and distribu- 

 tion of the rocks, the physical geography 

 of the time. The subdivisions of that 

 chapter of the 'Manual' are characterized, 

 not as series, systems and groups of strata, 

 but as eras, periods and epochs of time. 

 The common use, in recent geological 

 writings, of such phrases as 'Silurian era,' 

 rather than 'Silurian system,' etc., is a 

 testimony to the influence of Dana's mode 

 of treatment. 



II. THE THEOEY OF MOUNTAIN-MAKING 



The Geological Survey of Pennsylvania, 

 to which I have already referred, made 

 known the folded structure — the alternate 

 anticlines and synclines — of the Appa- 

 lachians. The beautiful sections of these 

 folded strata, in the Atlas of that survey, 

 reveal the thoroughness with which the 

 structure of the mountains was investigated 

 by Henry D. Eogers; and can be studied 

 with delight to-day, in spite of the fan- 

 tastic nomenclature, which the student has 

 to translate into the familiar language of 

 the New York Survey. In those sections 

 it appears that, in general, each fold is 

 unsymmetrical, the dip on the northwest 

 side being steeper than that on the south- 

 east, if, indeed, the dip on the northwest 

 side is not carried beyond the perpendicu- 

 lar and reversed. While in each fold, as 

 a rule, the dip is steeper on the northwest 

 side than on the southeast, if we compare 

 the successive folds, we find the dips grow- 

 ing more gentle as we go from southeast 

 to northwest. The nearly vertical or over- 

 turned dips of the folds on the eastern 



border pass by a gradual transition into 

 gentle undulations on the western border 

 of the Appalachian zone. 



While the stratigraphy was worked out 

 so beautifully in the first Geological Sur- 

 vey of Pennsylvania, the dynamic concep- 

 tion derived from it was crude indeed. 

 The conclusions of the author are summed 

 up in the following sentences : ' ' The wave- 

 like structure of undulated belts of the 

 earth's crust is attributed to an actual 

 pulsation in the fluid matter beneath the 

 crust, propagated in the manner of great 

 waves of translation from enormous rup- 

 tures occasioned by the tension of elastic 

 matter. The forms of the waves, the close 

 plication of the strata, and the permanent 

 bracing of the flexures, are ascribed to the 

 combination of an undulating and a tan- 

 gential movement, accompanied by an in- 

 jection of igneous veins and dykes into the 

 rents occasioned by the bendings. This 

 oscillation of the crust, producing an ac- 

 tual floating forward of the rocky part, has 

 been, it is conceived, of the nature of that 

 pulsation which attends all great earth- 

 quakes at the present day." The wave- 

 like form of the Appalachian anticlines 

 and synclines is a beautiful generalization 

 of accurate and conscientious observation; 

 but the dynamic theory suggested for its 

 explanation needs to-day no other refuta- 

 tion than its simple statement. 



But, however completely the Pennsyl- 

 vania geologists failed to construct a satis- 

 factory theory of mountain-making, their 

 observations of Appalachian structure were 

 of immense value in their destructive effect 

 upon some of the notions of mountain- 

 making prevalent at the time. In the text- 

 books in the early part of the nineteenth 

 century, a diagram often appeared repre- 

 senting a transverse section of an ideal 

 mountain range. A vertical wall of crys- 

 talline rock forms the center and the crest 

 of the range. Against this wall, on each 



