MINERALOGY. 73 



as occurring at Franconia. Massive garnet in large pieces is found here, 

 in the geodic cavities of which beautiful blood-red crystals are found asso- 

 ciated with calcite and magnetic iron. The following is an analysis, by 

 Fisher, of a nearly black specimen from this locality : 



Silica, 38.85 



Iron sesquioxide, .......... 28.15 



Lime, 32.00 



99.00 



The analysis shows, as he remarks, that it is a pure iron lime garnet, 

 remarkably simple in composition, the analysis of which corresponds 

 very exactly with the formula. 



The most beautiful garnets that have been found in New Hampshire 

 are perhaps the cinnamon or alumina lime garnets which are found 

 associated with green pyroxene at Warren and at Amherst. They are 

 of a cinnamon-brown color ; and the ones at Warren are very perfect in 

 their form. The Amherst crystals reach a very large size, some of them 

 being three or four inches in diameter ; but when so large they do not 

 often have perfect and smooth crystalline faces. These crystals are 

 found in the limestone, and also in the crystalline rocks, at their surfaces 

 of contact with the limestone. They are mostly simple dodecahedrons. 

 They are associated with pyroxene and vesuvianite, which are other lime 

 and alumina minerals. Dr. Jackson, in describing these rocks, has fol- 

 lowed some others in thinking that the occurrence of these lime silicates 

 at the junction of the limestone with the siliceous rocks, or primary rocks, 

 as they were termed, is sufficient evidence to prove that all these rocks 

 were of igneous origin, since the siliceous rocks on eruption would 

 inevitably generate such silicates of lime and alumina as garnet and 

 vesuvianite, on coming into contact with the limestones. But this is 

 not a weighty argument. Garnet and vesuvianite are minerals which, 

 although unaffected by acids in fine powder, are decomi^osed by hydro- 

 chloric acid with ease if they are ignited before treatment, and the solu- 

 tion, if evaporated, will gelatinize. This shows that some chemical 

 change is effected by heating a garnet, and therefore a garnet would 

 not be likely to be formed at a high temperature; and, as is well known, 

 it is not often found as a constituent of igneous rocks, save as a product 

 VOL. IV. 10 



