506 BARKER'S ANALYSES. 



" This resembles more closely the analysis of the Roxbury specimen, the silica being a little less and 

 the alumina a little more than in that rock. The proportion of the composing mineral varies, being mixed 

 with extraneous matter. Scattering through it are octahedral crystals of magnetic oxyd of iron. These 

 were carefully removed by the magnet from the portion of the rocks submitted to analysis. The presence of 

 these rendered the determination of the specific gravity inaccurate. It is less friable than the preceding ; 

 has more tenacity; before the blowpipe it is unaltered ; a thin sliver, however, being softened on the edges." 

 The geological position of this specimen is at the eastern part of the formation; and is probably not far 

 from the base. The rock is intermediate in character between clay slate and talcose schist. 



" Novaculite, Irasburgh. This is a mineral, very homogeneous in composition, fracture rough, structure 

 amorphous, translucent, of a dirty gray color, streak white, specific gravity 2.65. A careful analysis gave 

 the following composition : 



Oxygen. 



Silica, 



Alumina, 



Peroxyd of iron, 



Lime, .... 



Magnesia, . 



Potassa, 



Soda, 



Loss by ignition, . 



99.79 



" This would make the oxygen ratio, 1 : 3 :21, or (RO, R,0 3 ) 7Si.O 3 .. 



" Though called on the label talcose schist, its physical and chemical characters are widely different. Of 

 magnesia there is but a trace, and the loss by ignition is small. Before the blowpipe it becomes white 

 and opaque, is infusible, and colored the flame-yellow ; with cobalt solution it gives a fine blue." 



This is the most unctuous of all the specimens analyzed, and according to this supposed criterion for the 

 presence of magnesia, a large per cent, is indicated. A rock of this character is very common in the talcose 

 schist formation, and in the catalogue of the Vermont State Cabinet it receives the name of novaculite 

 schist. It is also abundant in the Georgia group of slates. 



Observing that upon the Canada geological map (in manuscript) the same color was given to the red 

 sandrock about Burlington, Vt., and to the talcose schist formation (which they call lower Silurian), we 

 were led to desire a chemical examination of the red sandrock. A specimen which had rather more silica 

 than common was used, with the following result ; analyzed by Mr. Barker : 



"A hard, compact sandrock ; specific gravity, 2.65 ; color, dark red, on ignition it changed color, becoming 

 blacker ; composition : 



Silica, 83.30 



Alumina and peroxyd of iron, . 8.70 



Lime, 1.12 



Magnesia, .10 



Potassa, 4.59 



Soda, .45 



Loss by ignition, ... .80 



99.06 



" The excess of silica is due to sand present. It could not be separated, as the rock is not decomposable 

 by acids. The oxygen ratio of the basis is 1 :3." 



With the exception of the excess of silica, which is only occasional, both in the red sandrock and talcose 

 schist, the proportions of the other components agree. Hence it is not unlikely that the sandrock may be 

 altered into talcose schist ; so that we may have two sources for the schist, from clay slate and sandstone. 



There is another consideration connected with this subject, of great importance. Associated with the 

 schists is gneiss. The two rocks pass into each other by degrees. For instance, the belt of aluminous 



