236 Mr. T. S. Hunt on the Composition and Metamorphoses 



of iron from 3 to 4. These specimens also contained, besides 

 minute quantities of lime and magnesia, from 10'5 to. 11 '5 per 

 cent, of alkalies in which the soda varied from a trace to 3"6 per 

 cent. Other specimens afforded 8 to 12 per cent, of lime, 7 to 

 8 of magnesia, and from 4 to 6 per cent, of potash, with traces 

 of soda ; the proportions of silica, water, and oxide of iron being 

 the same*. 



According to Prof. Shepard, this material is often associated 

 with the deposits of specular iron ore in the crystalline limestones 

 and felspathic rocks, which in northern New York underlay the 

 Lower Silurian strata, and are by Dr. Emmons of the New York 

 Geological Survey, regarded, the limestones and iron included, 

 as hypogene rocks. The researches of Mr. Logan upon the ex- 

 tension of these rocks into Canada, where they are very widely 

 spread, have, however, shown them to be undoubtedly altered 

 sedimentary deposits, leading us to infer in both cases a similar 

 origin for these hydrous alkaline silicates. 



The large amount of alkalies in the schists of St. Nicolas is 

 worthy of notice, because the small quantity remaining in kaolin 

 and some other clays seems to have given rise to an idea that 

 such sedimentary rocks are generally deficient in alkaline ingre- 

 dients. Another red slate from the Etchmin river, belonging 

 to the same strata as the last, afforded the following analytical 

 results : — 



Silica 66-00 



Alumina and protoxide of iron 24*60 



Potash 3-67 



Soda 2-23 



Lime and magnesia .... traces 

 Water 3-00 



99-49 



Although more siliceous, it resembles that of St. Nicolas in 

 the large proportion of alkalies and the predominance of potash. 

 A glance at the first analyses will show that that slate under 

 igneous action might produce a large amount of a potash felspar 

 like orthoclase, together with an alumino-ferruginous augite ; it 

 is, in fact, the almost unchanged detritus of old felspathic and 

 pyroxenic rocks, and is converted into similar rocks in the meta- 

 morphic region further south. 



The following analyses were made for the purpose of deter- 

 mining the chemical composition of good roofing-slates, but are 

 not without value to the chemical geologist. No. L is from 

 Kingsey in the Eastern Townships, and, like those already dc- 



* American Journal of Science (2), vol. xvi. p. 50. 



