328 ^Review of 'Recent Geological Literature. 
through the centers of the bases, the spirals will assume the position 
that characterizes the genus Dayia Dav. Continuing the rotation their 
position in Thecosfira Zag., is soon reached, where they point to the 
bottom of the ventral value. When a half turn has been made and the 
spirals point toward each other in the center of the shell the position 
characterizes the genus Glassia Dav. After 225 degrees have been 
passed over and the spirals are directed obliquely into the dorsal value, 
Zy^ospira is indicated, and lastly Ai'iyfci, Avhich has the spirals directed 
to the bottom of the dorsal valve, is represented after a turn of 270 de- 
grees.' 
T/ie Taconic of Georgia, and the report on the geology of Vermont. By 
Jules Marcou. (From the memoirs of the Boston Society of Natural 
History, vol. iv. Read May 4, 1887.) This valuable paper gives some 
further facts respecting the geology of northwestern Vermont, and 
sketches the historj' of the term " Georgia formation." Since the death 
of Dr. Emmons the burden of the pro-Taconic discussion has been ably 
maintained by Mr. ISIarcou, and he welcomes the advent of the new re- 
searches and conclusions of Mr. Walcott. •' I salute with joy the arrival 
of new observers, better fitted ovit and armed than I was, sure that now 
the truth will not be kept much longer in the background." Mr. Marcou 
concludes that the term Lorraine shales should be tisCd for " Hudson River 
group," which has become so hopelessly involved. In this he is followed 
bv Mr. S. W. Ford (Am. Jour. Sci. xxix, p. 16). The age of the red 
sandrock near Swanton he considers still to be somewhat puzzling. He 
had taken it to be the Potsdam, wholly, but the discovery of the fauna 
of the Georgia slates in some part of it by Mr. Walcott, he thinks shows 
it is older than the Potsdam. That is to sa^', that Mr. Walcott has dis- 
covered the middle primordial fauna in it, whereas it seems that Mr. 
Marcou regards the Potsdam as containing the upper primordial fauna 
— an assumption which has not yet been established. It is perhaps as 
likely that the New York Potsdam, when fully examined, will be found 
to contain the middle as the upper primordial. The " Potsdam fauna" 
' The relation here pointed out by Mr. Glass is exceedingly interesting 
to the pala;ontologist, and may be thus represented if we begin with the 
position of the spirals in Athyris as described above. 
Athyris 0° } 135° Atrypa 270° 
Dayia 45° Glassia 180° ? . .'. 315° 
Thecospira 90° Zygospira 225° Athyris 360° 
Where are the genera indicated by the interrogation points, and where 
are the missing links between the genera already defined.^ Probably 
more careful measurement of the angles will bring some of them to 
light and aid in filling the series. 
Similar continuous relations will probably be traced in the structure and 
position of the loops or calcareous fibers connecting the spirals of which 
Mr. Glass adds descriptions, as they occur in the leading genera of 
brachiopods. . 
