672 PROCEEDINGS OF THE BALTIMORE MEETING 



cieut, birefringence, and circular polarization all changing abruptly. O. Miigge 

 (Neues Jahrbuch, Festband, 1907, 181-196) has recently considered the problem 

 again in detail, and by means of etch figures combined with crystallographic 

 behavior on heating found that below the inversion point quartz crystallizes in 

 the trapezohedral-tetartohedral division of the hexagonal system, while above 

 570 degrees it is trapezohedral-hemihedral. The high form is very similar to 

 the low form, and differs chiefly in the fact of its common planes of symmetry. 

 A plate formed above 570 degrees is trapezohedral-hemihedral, but on cooling 

 it changes to the trapezohedral-tetartohedral division, thereby losing its com- 

 mon planes of symmetry, which may then become twinning planes. It is to be 

 expected, therefore, that quartz crystals thus cooled will be irregularly and 

 intricately twinned after (1010.), while low temperature quartzes are simple 

 or regularly twinned. It is furthermore evident, on considering the genesis of 

 quartz at different temperatures, that intergrowths of right and left handed 

 quartz are limited chiefly to quartz crystals formed below 570 degrees. These 

 two criteria can be used to distinguish quartz which has been formed or heated 

 above 570 degrees from quartz which has never reached that temperature. 

 The object of the present investigation has been to test the general validity of 

 the theoretical conclusions on a number of quartzes from different kinds of 

 rocks and veins, as well as to determine more accurately the inversion tempera- 

 ture. 



Session of Thursday, December 31 



The sectional meetings for papers on stratigraphic, areal, and paleon- 

 tologic geology was called to order at 10 o'clock Thursday morning by 

 W. B. Clark, who was then elected presiding officer. E. E. Cumings 

 acted throughout as secretary by request of the Secretary of the Society. 



The first paper read was 



OCCURRENCE OF THE MAQOTHY FORMATION ON THE ATLANTIC ISLANDS 

 BY ARTHUR BARNEVELD BIBBINS 



[Abstract] 



The Magothy formation (of mid-Cretaceous age), as originally defined by 

 Darton, was supposed by that author to be limited to the state of Maryland, 

 although its partial equivalent, the "alternate clay-sands," was earlier men- 

 tioned by Uhler as occurring much farther northward. Recent investigations, 

 paleobotanical and stratigraphic, by Hollick, Berry, and the writer, have ex- 

 tended the lines of the formation far southward, and northward across New 

 Jersey and along the Atlantic islands as far as Marthas Vineyard. The occur- 

 rence on these islands was shown by local sections and photographs. The 

 deposits are richly plant bearing, with grains of amber associated, as on the 

 Magothy river. The formation suffered considerable corrugation by the great 

 ice-sheet. 



The paper was discussed by David White and A. B. Bibbins. 



