LITHOLOGY. l6l 



times quite large crystals of titanic iron. Under the microscope, with 

 polarized light, feldspar crystals of some size are seen, but they are so 

 impure that they can scarcely be distinguished from the ground mass 

 without polarized light. In some specimens, however, they are pure 

 enough to see all their bandings ; and in appearance and mode of decom- 

 position they much resemble the anorthite of the diabase, and are en- 

 tirely unlike the feldspar of the diorites next to be described. At Camp- 

 ton falls there are several dykes which furnish handsome specimens for 

 those who admire dark, porphyritic rocks. The black crystals of horn- 

 blende are not large enough to determine with the unaided eye, but they 

 are very brilliant and numerous. The following analysis made by Mr. 

 Pease, indicates the general composition of this rock: 



Silica, 43.39 



Alumina, 15-85 



Iron sesquioxide, 6.56 



Iron protoxide, .......... 9.51 



Lime, 9.47 



Magnesia, 5,01 



Soda, 5.15 



Potash, .19 



Carbonic acid, .......... 2.47 



Water, 3.29 



100.89 



This analysis is interesting as showing the very basic nature of these 

 rocks, but beyond that, little can be deduced from it, though it may be 

 suspected that the feldspar is basic too ; but the predominance of horn- 

 blende, the presence of much magnetite or titanic iron, and the compli- 

 cated nature of the rock, leave any conclusion that may be drawn only 

 probable. 



When examined under the microscope, beyond the general character- 

 istics of the rock already mentioned, some interesting details are brought 

 to view. The hornblende crystals are deep brown when the light passes 

 through them in one direction, and bright yellow when it passes at right 

 angles thereto. They are often twins, the twinning plane being parallel 

 to the orthodiagonal. Moreover, some of these crystals are hollow, and 

 the cavities that they contain have a contour like the exterior, and are 

 VOL. IV. 21 



