2ao 



SCIENCE. 



[X. S. Vol. II. Ko. 36. 



The succeecling speaker was A. Cajjen 

 Gill, of Ithaca, N. Y., on ' A Geological 

 Sketch of the Sierra TIayacac, in the State 

 odT Morelos, Mexico. The Sierra TIayacac, 

 aitaated to the southward of the great fault- 

 line described by Felix and Lenk, consists 

 of a projecting group of mountain tops in 

 the midst of the Morelos Plain. The plain 

 is formed by the lava streams and ejecta- 

 menta of Popocatepetl or neighboring vol- 

 canic vents. The tops of the nearly sub- 

 m.ei'ged mountains show that the fol ding and 

 elevation of the Cretaceous (Caprrua ?) lime- 

 stone was accompanied or followed by the 

 deposition of a limestone conglomerate, in 

 the pebbles of which are also Caprina (?) 

 fbssUs. Lack of eo-uptive pebbles indicates 

 that the volcanic activity of the region was 

 subsequent to extensive folding and erosion. 



The limestone conglomerate is overlain 

 by an acid eruptive, and both rocks are cut 

 by numerous dykes which show a close 

 ' consanguinity ' with the recent extru- 

 sions of Popocateptl. The very striking 

 metaphorphism produced by these dykes 

 corroborates the view that there is little, if 

 any, migration of material from the intruded 

 mass into the metamorphosed rock. 



Heated water and steam would appear 

 to be the principal agents of metamorphism, 

 rather than heat alone, since the great dis- 

 tance to which recrystallization has reached 

 seems dependent on the porous character of 

 the rock beibre alteration. 



Garnet, vesuvianite, wollastonite and py- 

 roxene are among the minerals developed, 

 and large crystals have been found at a dis- 

 tance of several hundred feet from the con- 

 tact. 



Considerable discussion followed in which 

 the forbearance of the author in refraining 

 from the creation of new rock-names, was 

 heartily commended. 



The session then adjourned until 9 A. M. 

 of Wednesday. On reassembling the fol- 

 lowing morning the first paper was pre- 



sented by W. M. Davis, Cambridge, Mass., 

 on ' The Bearing of Physiography on Uni- 

 formitarianism. ' 



The conditions and processes postulated 

 in the phj'siographic study of land forms — 

 geomorphologj' of some authors — are among 

 the cardinal principles of uniformitarianism. 

 The success in the interpretation of nature 

 by means of this kind of study confirms the 

 correctness of its postulates, and thus brings 

 to the support of uniformitarianism a large 

 class of facts, whose bearing on this theory 

 was not at all perceived when its earlj' ad- 

 vocates announced it. These general prin- 

 ciples were foi'ther elucidated by the exam- 

 ple of the development of the river Marne 

 in northeastern France, and of its associ- 

 ated streams. The migration of divides 

 and the robbing of one stream bj^ another 

 in the course of slow degradation were 

 traced out as an illustration of large effects 

 from the operation of slow and gradual 

 causes. In discussion B. K. Emerson cited 

 similar cases of the robbing of one stream's 

 headwaters by another, in the relations of 

 the Housatonic and Connecticut divides in 

 western Massachusetts. President Shaler 

 emphasized the importance of continental 

 tilling in bringing about these changes of 

 drainage, and illustrated his point by cases 

 in the Berkshire Hills. 



C. E. Van Hise, of Madison, Wis., fol- 

 lowed with a paper on the ' Analysis of 

 Folds.' As regards movement three zones 

 in the constitution of the earth were cited, 

 an outer of fracture, an inner of fracture 

 and flowage, and an interior one of flowage 

 alone. The particular depth or extent of 

 each depends on the hardness or softness of 

 the strata; shales, for instance, flow at a 

 small depth. The subject of folds was then 

 taken up, and it was shown that the ordi- 

 nary treatment of the subject with sections 

 in only two dimensions was incomplete in 

 that it failed to properly emphasize the 

 pitch of the axes and the presence of other 



