58 MINERALOGY AND LITHOLOGY. 



which are parallel to the ordinary cleavages of that mineral. If, now, we 

 double the length of the orthodiagonal, and connect the ends with the 

 ends of the same clinodiagonal, we obtain the base of the hornblende 

 prism to the sides of which the ordinary cleavage of hornblende is par- 

 allel. Now, in these altering grains of augite in this sienite from Jack- 

 son, the cleavages of these two minerals bear the exact relationship to 

 one another that these two figures do when thus constructed, as is shown 

 in PI, 7, Fig. i. This is a basal section, and the cleavage of the outside 

 hornblende is seen to be parallel to the outside figure of the accompany- 

 ing diagram, while the inside augite exhibits a cleavage parallel to the 

 inner part of the little diagram. The hornblende is strongly dichroic, 

 as is shown by the yellow bit above, which is cut parallel to the prism, 

 while the augite is not dichroic. This case of alteration furnishes a most 

 instructive illustration of the relationship that exists between these two 

 minerals. 



Another kind of change has been effected by that slow weathering 

 that has converted the pyroxene into a green hydrous mineral. Pyrox- 

 ene is very subject to this kind of alteration. This green product is 

 known to be a kind of chlorite, and viridite is a name that has been pro- 

 posed for it when its nature is unknown. This is a convenient word to 

 apply to the green unknown results of decomposition ; but several persons 

 have attempted to determine the nature of this viridite, which plays so 

 important a role in basic rocks, and Dr, K. L, Th, Liebe determined it 

 to be a kind of chlorite, with the composition of an unisilicate, a con- 

 clusion confirmed by an analysis made by myself on pure material gath- 

 ered from diabase. Now, the change from the calcareous bisilicate 

 pyroxene to the magnesian unisilicate chlorite involves the separation 

 of a definite amount of lime and silica, and hence, as a rule where this 

 decomposition has taken place, wc find lime carbonate and silica as asso- 

 ciations of the green chlorite. Other kinds of decomposition take place, 

 resulting in the production of epidote and various hydrous silicates ; and 

 sometimes augite crystals are decomposed into a heterogeneous mixture 

 with the mere outline preserved. 



41. Rhodonite [Mn Si O3]. 

 Rhodonite is abundant in some localities in the south-western part of 



