318 GEOLOGICAL APPEARANCES 



sei'pentine in one or two places only. Almost all these com- 

 pounds effervesce, more or less, with muriatic acid ; and of 

 course carbonate of lime, or carbonate of magnesia, perhaps 

 both, are among their ingredients. Small specks of iron py- 

 rites are frequent in them. One of the most common of them 

 consists of quartz and compact dolomite. he quartz is in a 

 large proportion ; but the dolomite renders the mass extreme- 

 ly tough under the hammer, and imparts a little of its charac- 

 ters to the fracture, which sometimes approaches to that of 

 compact felspar. It effervesces with muriatic acid, though of- 

 ten feebly. Its colour is whitish-brown, where it is not de- 

 composed. By decomposition, it assumes a rusty-brown colour 

 near the surface, indicating perhaps that manganese is an in- 

 gredient. Specimens from different spots present many varie- 

 ties in these general characters. In some of the rocks consist- 

 in"' of the compounds above mentioned, neither the form of a 

 bed, nor a stratified structure, is discernible ; but we were led 

 to consider them as parts of the strata, from the imperfect 

 crystallisation of their ingredients, and their analogy to others 

 that are distinctly stratified. 



37. The rocks of sienite do not indicate stratification, either 

 by the form of a bed, or by the structure of the stone. Of the 

 sienite there are two varieties, one grey, and the other red ; the 

 colour of each depending on that of the felspar, which in both 

 is the principal ingredient. Both contain quartz, and agree in 

 most other circumstances. We paid little attention to the 

 orey, as we no where found it in contact with the strata ; but 

 the characters, which distinguish it from the grey sienite gra- 

 duatin<T into sienitic greenstone, on the northern side of the val- 

 ley, have already been mentioned. In the red sienite, the fel- 

 spar presents different shades of red, graduating into grey ; the 

 hornblende varies much in its proportion, and in the size of its 



grains, 



