156 MINERALOGY AND LITHOLOGY. 



PI. 5 in Fig. 3. The rock contains augite of a light pink color, much 

 green chlorite, and a little biotite. The iron oxide is titanic iron, and 

 its solid centres are usually surrounded by a gray, translucent rim. Apa- 

 tite is abundant. 



The massive portion of this rock has been analyzed by Mr. Pease, of 

 the Sheffield laboratory, with the following result : 



Silica, 47.38 



Alumina, 19.08 



Iron sesquioxide, 2.66 



Iron protoxide, 8.81 



Lime, 8.37 



Magnesia, 6.07 



Soda, 3.54 



Potash, 1. 31 



Water, 3.39 



Carbonic acid, .79 



101.40 

 Specific gravity, 2.90. 



An analysis of the anorthite from this rock has been given on page 91, 

 which indicated the composition of the aggregate into which the large 

 crystals have been converted. But the feldspar in the compact part of 

 the rock is not so altered as are the large anorthite crystals; and the 

 analysis points towards a soda lime feldspar like labradorite, and makes 

 it probable that two kinds of triclinic feldspars are present, as has often 

 been proved to be the case elsewhere. 



A specimen from Moose Mountain, and another from Stark, offer no 

 further peculiarities, save the presence in them of much pyrites. A 

 specimen from Concord, Vt., contains much calcite, and the anorthite is 

 almost entirely altered into an aggregate. 



Besides the occurrences of diabase porphyries that have been men- 

 tioned, there arc a great many others in which the feldspar has reached 

 such a state of decomposition that neither analysis nor the microscope 

 can determine its species. In regard to all these, it may be said, that 

 whether originally labradorite or anorthite is now of little consequence, 

 since time has reduced one and the other to the same thing, and for such 

 rocks porphyritic diabase is a name sufficiently satisfactory. It may be 



