December 18, 1806.] 



SCIENCE. 



927 



ture are developed. The rocks are very much 

 deformed, by folding, faulting and metamor- 

 phism. The two former are as extreme as in any 

 part of the Appalachians, and the metamorphism 

 is unusually developed. The principal fault, 

 which constitutes the'essential feature to be con- 

 sidered, almost encircles an area of Cambrian 

 rocks which are overthrust by granite. The 

 positions and relations of the formations were 

 pointed out on a map of the district. Mr. 

 Keith stated that in the processes of metamor- 

 phism the diabases became schists and the 

 gi'anites gneisses. One of the chief features 

 developed is the offsetting of the different for- 

 mations on opposite sides of a shear zone ex- 

 tending nearly west across the strike. 



The great question was, how the deformation 

 of the basement rocks occurred. He mentioned 

 two theories. One is that the sediments above 

 were thrust and defined against the crystallines. 

 This theory he combatted as being untenable. 

 The other theory is that the force that produced 

 the deformation originated in the crystallines 

 and was propagated toward the northwest. 



The subject was discussed by Messrs. Hayes 

 and Willis. 



Mr. George F. Becker read a paper, en- 

 titled 'Some Queries on Rock Differentiation.' 

 The theory now almost generally received as- 

 cribes the origin of many igneous rock species 

 to the segregation of a single homogeneous 

 magma under differing physical conditions. 

 Mr. Becker considered segregations clue to dif- 

 ferences of temperature (LudAvig's or less 

 properly Soret's method) and segregation 

 caused by varying pressure. The active force 

 in these processes is osmotic pressure and 

 this produces segregation by ' molecular flow.' 

 Molecular flow, one case of which is ordi- 

 nary diffusion, is an exceedingly slow process 

 when the distances involved are considerable; 

 even an aqueous solution of copper sulphate 

 diffuses in sensible quantity to no more than 

 35 cm. in a year and to ten times this distance 

 in 100 years. Temperature in underground 

 strata diffuses more than 4,000 times as fast 

 as this sulphate. Mr. Becker thinks the pro- 

 cess of segregation too slow to produce the 

 observed rock differences. Ludwig's method 

 involves the heating of masses of magma from 



the top. If such a case were to occur in 

 nature, and this could only be under very 

 exceptional conditions, the temperature would 

 be equalized by conduction through the walls 

 before considerable segregation had had time 

 to take place. " Differences of pressure would 

 produce only a very slight segregation even 

 after an infinite time. The separation of 

 a magma into immiscible fractions under 

 changes of temperature was also considered. 

 This involves a superheating of the magma, 

 which seems impossible in the presence of solid 

 rock. It also implies very fluid magmas ; too 

 fluid, for example, to hold phenocrysts in sus- 

 pension. In a superheated magma a decrease 

 of pressure would promote miscibility, not 

 separation. Mr. Becker suspects that the dif- 

 ferences between the main rock types is due to 

 the original heterogeneity of the solar nebula, 

 and that many rocks are mere fortuitous mix- 

 tures originating at the contacts of primordial 

 masses. 



The paper will be printed in the January 

 number of the American Journal of Science. 



Mr. Whitman Cross was on the progi*am 

 of the meeting for a communication on Land-* 

 slides in Colorado, but for lack of time the 

 paper was deferred until the next meeting. 



W. F. MOESELL. 



United States Geological Survey. 



academy of natural sciences of phila- 

 delphia, november 24. 



Mr. J. H. Hamilton recorded his recent 

 discovery of what may prove to be an unde- 

 scribed trap dyke in Fairmont Park, on the line 

 of the newly opened trolley railway, about 175 

 feet from the Elm Avenue entrance. The trend 

 of the trap is N. by 5°E. 



Mr. Eichardson stated that he had found, 

 detached, not in situ, masses of zoosite at the 

 same locality. 



December 1, 1896. Mr. Theodore D. Rand 

 gave an account of a recent trip made to the 

 Wilmington gabbro region, and described the 

 distribution of the quarries as reported by Prof. 

 Chester and as throwing light on the geology of 

 Delaware and Pennsylvania. He referred speci- 

 ally to a mass of the gabbro on the Brandy wine 

 Creek, just below the Jessup and Moore paper 



