SKY. GEOLOGY. GNEISS. 



of these portions of. calcareous rock containing the same 

 oblique gryphite ammonita, and terebratula. These strata 

 are lastly followed by another set of stratified rocks, con- 

 sisting of calcareous sandstone, limestone, and shale ; the 

 whole being intersected and succeeded by different mem- 

 bers of the trap and syenite family in the usual irregular 

 and overlying position.* The arrangement thus briefly 

 detailed is in some respects strictly geological, while in 

 other points it will be found fallacious, as will be shown 

 in the course of the ensuing detail. 



In entering on the description of gneiss as the lowest 

 rock, it must be premised that I have chosen this term 

 as a general one, to prevent repetition or prolixity where 

 general features only were to be described. Respecting 

 the varieties included under it, I need add nothing to 

 the general remarks formerly made on this subject; but 

 in describing the individual beds of this series, I have 

 with mineralogists limited the term gneiss to its strict 

 use ; distinguishing by a separate description those sub- 

 stances which differ from it in structure or composition, 

 and giving to the alternating rocks of other characters 

 their received appellations. 



In choosing the most obvious character as the criterion 

 of the upper or lower parts of any inclined series of beds ; , 

 it is taken for granted, that no set of strata has been 

 so entirely changed from its original position as to have 

 passed the perpendicular and acquired the opposite dip. 

 But in the present case the upper portions are in reality 

 determined by the rocks that follow, and we may therefore 

 conclude the rule to be here inapplicable ; since the lowest 

 portions of this gneiss are the uppermost in geological 

 order, being in contact with those strata which are known 

 from other circumstances to succeed in the order of super- 

 position. It is nevertheless convenient to commence the 

 description from this part of the series, 



* Plate XIV. fig. 1. 



