Fairbanks.] 



A ualcitc Diabase. 



295 



isolating portions of the same augite individual. The feldspars 

 show twinning (albite law) and high angle of extinction in a man- 

 ner similar to that of the Cuyamas dike. Tabular sections are less 

 abundant, lath-shaped forms predominating. But few feldspar indi- 

 viduals have a fresh appearance, having been changed to kaolinic 

 matter or penetrated and largely replaced by green decomposition 

 products, originating in the olivine and angular analcite areas. A 

 replacement of the feldspars by analcite has taken place in a man- 

 ner similar to that before described, but it is less pronounced. This 

 change is more noticeable in the fresher individuals. 



The augite is similar to that before described, but more rarely 

 shows crystal forms and never that distinct orthopinacoidal cleav- 

 age. In most specimens it is the freshest component of the rock. 



To the decay of the olivine is due in part the alteration of the 

 other components. All sections show areas of green matty and 

 granular material, which, from the frequent hexagonal forms with 

 clear centers and dark borders, filled with magnetite granules, and 

 very pronounced reticulated structure, are judged to represent 

 olivine. The centers are in many cases brilliantly polarizing and 

 intersected by clear rectangular lines. The decomposition products 

 are serpentinous in character. 



There are other sharply angular areas between the feldspars 

 filled with green matty and fibrous material, which, it is believed, 

 has replaced analcite, as in places that isotropic mineral still remains. 

 (Plate 16, Fig. 5.) On account of the decomposition of the analcite 

 filling these wedge-shaped areas, it was distinctly determined in 

 only two slides, but it was undoubtedly present at one time in all. 

 The freshest portion of the dike has a very peculiar structure, 

 crumbling to spherical bodies fifteen to twenty millimeters in 

 diameter. Titaniferous magnetite is present in this dike, as before 

 described, in the form of grains and long rods. 



In the dark and fine-grained types confined to the three east- 

 erly dikes, as in the one just described, it is often impossible to 

 obtain coherent specimens in place, and the exact position in the 

 dike of many studied is unknown. Nearly all the sections made 

 from these rocks show a much fresher condition of the feldspar and 

 olivine. The former are more lath shaped, and high angles of 



