DOLERITE INTO HORNBLENBE-SCHIST. 137 



between the dolerite and the hornblende-schist, so far as specific 

 gravity is concerned. A bulk-analysis of the schist also shows a 

 close agreement with that of the dolerite : — 



SiO, 49-78 



TiO, 2-22 



Al,03 33-13 



FeA 4-35 



FeO 11-71 



MnO -27 



CaO 8-92 



MgO 5-40 



K,0 1-05 



Nap 2-39 



CO, -10 



Hp 1-14 



100-46 



The slightly greater percentage of silica in the schist is exactly 

 what the microscopic analysis would lead one to expect. It would 

 be interesting to know whether this is a constant feature, and, if so, 

 whether the silica has been introduced from without, or whether 

 bases have been removed. The isolation and separate analysis of 

 the individual constituents of the two rocks would probably lead to 

 interesting results. The fact that the schist contains more iron in 

 the ferric state, suggests that an oxidation of a portion of the iron 

 occurs in the change from augite to hornblende. This would effect 

 the liberation of a certain amount of silica ; and it is interesting to 

 note that in certain granular varieties of the rock, in which horn- 

 blende occurs to the exclusion of augite, detached grains of a clear 

 mineral resembling quartz may frequently be seen scattered through 

 a plate of hornblende having definite optical properties. 



The constituents of the rock are hornblende, quartz, felspar, 

 or minerals resulting from the modification of felspar, titaniferous 

 magnetic iron-ore, sphene, and apatite. The rock is to all intents 

 and purposes holo-crystalline and possesess the true micro-structure 

 of a schist. 



The hornblende occurs in irregular grains, which possess definite 

 optical properties, but are without external crystalline faces. They 

 are usually of unequal dimensions in the different directions, and the 

 corresponding axes of the diff'erent grains lie roughly, but not rigidly, 

 parallel to each other in the rock. The longest diameter of a grain 

 usually corresponds approximately to the vertical axis of the crystal, 

 and lies in the plane of schistosity ; the mean diameter corresponds 

 with the orthodiagonal, and also lies roughly in the plane of schist- 

 osity ; the shortest diameter corresponds with the clinodiagonal, and 

 lies at right angles to the plane of schistosity. The general tendency 

 to this mode of arrangement is admirably brought out when a 

 section at right angles to the plane of schistosity and parallel to 

 the direction of arrangement of the longest diameters is compared 

 with one parallel to the plane of schistosity. On rotating the 



