MINERALOGY. 119 



78. Penninite. 



To penninite are referred those little hexagonal plates of chlorite that 

 are sometimes found in the chlorite schists, and which have the optical 

 properties of an uniaxial crystal. No chlorite of this form and composi- 

 tion has been found in condition for analysis, and it is hence only to be 

 borne in mind as being a probable constituent of our chlorite rocks. 



Quite different, now, are those chlorites that are so abundant in basic 

 rocks, being usually products of the decomposition of ferruginous min- 

 erals. One of the best determined of these minerals is, — 



79. Prociilorite [H^^ (Fe, Mg)'" Al« Si« O^'^]. 



This is a chlorite of an uncertain crystalline form, though often found 

 in hexagonal plates. It is not strictly uniaxial, however, and its optical 

 properties do not appear to be those of an hexagonal mineral. It is a 

 magnesia iron chlorite, which, according to Rammelsberg, has an atomic 

 ratio between the protoxide elements, the alumina (Ala) and silica of 3.3 : 

 1 : 2 as the mean of many analyses. 



On the road from the Glen house to the top of Mt. Washington there 

 are some large trap dykes ; and between these dykes and the surround- 

 ing rocks is a layer, six inches or more in thickness, of a pure crystallized 

 chlorite, which forms a selvage. It gave me, on analysis, — 



Silica, 25.32 



Alumina, 20.94 



Iron sesquioxide, .......... 1.94 



Iron protoxide, .......... 26.71 



Manganese protoxide, .20 



Magnesia, i4-05 



Water, 9.90 



99.06 



It is remarkable how very closely this analysis approaches to the atomic 

 ratio adduced for prochlorite, which proves conclusively that it is that 

 mineral. 



This chlorite is very deep green, and in thin sections is dichroic, though 

 not so strongly so as some other chlorites. Under the microscope it 

 appears unusually pure, its only inclusion being crystals of sphene. In 



