TALCOID SCHISTS. 425 



MAGNESIAN SLATE (Provisional) : Annual Report upon the G-eology of Vermont, 1845- 

 1848 ; by Prof. C. B. Adams. 



MAGNESIA^ SLATE : 'Appendix to Thompson's Vermont, 1853 ; by Prof. Zadock TJiompson. 



UPPER HUDSON" RIVER GROUP: Elementary Geology, 31st edition, p. 411, 1860; by 

 Edward Hitchcock and C. II. Hitchcock. 



TALCOID SCHISTS : The present Eeport. 



This group of rocks has generally been associated with the slates of the Georgia group, 

 having been regarded both as Lower Silurian and sub-Silurian, by different authors. 

 We propose to describe it as a separate group, although it may include rocks of different 

 ages. - The rocks thus specified form one great band, though somewhat broken by erosion, 

 extending from the south line of the State as far north as Pittsford. 



Litholoyical Characters of the Group. 



1. Talcoid schists are the prevailing rocks of the group, but the following varieties of rock are associated 

 with them : 



2. Talco-argillaceous schists, 



3. Talco-micaceous schists, 



4. Talcoid grits, 



5. Talcose conglomerates, 



6. Beds of clay slate, 



7. Beds of limestone. 



The most common rock of the group is what geologists have been in the habit of calling 

 talcose schist, and what Emmons calls magnesian slate, from the supposed presence of 

 magnesia. Much of this schist is of a light gray color, and much of it is of rather a green 

 color. A darker shade is sometimes imparted, by the presence of decomposing sulphurets. 

 It has an unctuous feel, like the genuine magnesian schists. A specimen of the common 

 variety of schist from Pownal has been analyzed, and the following are the proportions 

 of its constituent parts : 



Silica, 42.90 



Alumina and peroxyd of iron, 42.20 



Lime, 78 



Magnesia, . . . ' . 1.98 



Potassa, .... 5.24 



Soda, 1.33 



Loss by ignition, . . . 5.60 



100.03 



This is clearly an aluminous schist. It approaches the mineral dysyntribite in its compo- 

 sition, also the mineral talckite. The former is found in the Laurentian rocks, in Northern 

 New York, and the latter is found in Cambrian schists in Ireland. Owing to the absence 

 of magnesia in this rock, the name talcose is a misnomer. Accordingly we have adopted 

 the term talcoid schist to designate this bed of rocks. The name talcose is not yet gone 

 out of use, and is used for this rock in some parts of this Report. It is difficult to fix 

 upon any satisfactory definite term for this rock, in this transitional period of its history. 

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