48 HAEHL AND ARXOLD — THE MIOCENE DIABASE. [Feb. 5, 



cleavage was observed. Irregular patches of the enstatite were 

 found included in the feldspars. 



On the whole the pyroxenes are remarkably fresh. Many slides 

 show absolutely no decomposition products even where the feld- 

 spars are badly weathered. In a few instances slight chloritization 

 was noted, that being practically the only indication of decomposi- 

 tion. 



Olivine. — The olivine, like the augite, is present in two genera- 

 tions, porphyritic and microlitic. The porphyritic crystals are usu- 

 ally idiomorphic and are among the oldest segregations of the 

 magma. They are usually much fractured and jointed, and rounded 

 or embayed by the corrosive action of the magma. Usually disin- 

 tegration has gone on to such an extent that only the crystal form 

 remains, filled with the secondary products. Where the original 

 olivine remains it is cleay and colorless, with strong double refrac- 

 tion and high relief. It usually includes much magnetite in small 

 grains, some glass and dust particles. 



Fig. 24. Phenocryst of olivine (O) weathering to iddingsite (I), x 60. 



The most common product of decomposition is serpentine, which 

 frequently shows its fibrous character. Alteration begins along the 

 cracks, gradually working inward from them until the crystal is di- 

 vided into a number of irregular rounded patches of clear olivine 

 separated by fibrous serpentine, the whole making up the complete 

 form of the original crystal. Perhaps fifty per cent, of the pheno- 

 crysts studied were completely replaced in this way, the remainder 

 showing various stages of such decomposition or alteration in like 

 manner to the mineral, which is probably iddingsite (see Fig. 24 



