i8I 1.] hnperial InstUnle. 465 



overlying rocks in general. From a series of observations which 

 were made in the Highlands of Scotland, it appears that many of 

 the primitive overlying sienite, granite, and porphyry formations of 

 mineralogists, are not so in reality, but are thick confoimable 

 beds of these rocks which rise more or less above the sui,Tounding 

 strata. At the same meeting Professor Jameson described the 

 Criffle district of granite and sienite, situate in the county of Gal- 

 loway. Tiiese rocks occupy a considerable tract of country, and 

 rise to the heigiit of 1895 feet above the level of the sea ; (hey are 

 not distinctly stratified, and exhibit many interesting appearances, 

 of apparent fragments, of cotemporaneous veins, and transitions 

 into porphyry. Tiie rocks which rest immediately on the granite 

 or sienite are line granular compact gneiss, slaty sienite, horne- 

 blende rock, and compact felspar rock. These rocks alternate 

 with each other, and sometimes even with tlie sienite or granite ; 

 and cotemporaneous veins of granite are to be observed shooting 

 from the granite into the adjacent stratified rocks. At the Needle's 

 Eye on the west of Galloway, the Professor observed very fine ex- 

 amples of cotemporaneous veit)s and masses of granite, &c. in 

 compact slaty felspar ; and the felspar itself points out a hitherto 

 unsuspected connection of this mineral with certain kinds of clay- 

 slate. On these rocks rest greywacke and greywacke slate, and 

 jometimes transition porphyry; and it would appear, from Mr. 

 Jameson's observations, that there is an almost uninterrupted tran- 

 sition from the gneissy rock into greywacke ; and tliat when the 

 felspar of the greywacke increases very much in quantity, becomes 

 compact, dark coloured, and slaty, the greywacke at length passes 

 into clay-slate. 



IMPERIAL INSTITUTE OF FRANCE. 



Account of the Lnhours of the Class of Mathematical and Physical 

 Sciences of' the Imperial Institute of' France during the Year 1813. 



{Continued from p. 392.) 



New Memoirs on the Polarization of Light. By M. Biot. 



The memoirs read this year by M. Biot, contain the dcvelope- 

 ments and the conseqi;i nces of those of whicli we have given an 

 account in our preceding analyses. We cannot render our ac- 

 count of them inrelligiljle without noticing some of tlie fuels con- 

 tained in the preceding ones. M. Biot had first considered the 

 phenomena of coloration, first observed by J\I. Arago in thin 

 plates of certain crystallized and unerystallized bodies. These 

 phenomena are not represented by the formulas given by Malus for 

 polarization in Iceland crystal. M. Biot finds a general expression 

 which cmbniccs tliem all in every possible position of the plates. 

 He i-liows the relation which subsists between these colours |ola- 

 rized extraordinarily by these plates, and those of the thin plates of 

 unerystallized bodies which Newton had observed, and which fur- 



Voi.. 111. N" VI. 'J G 



