ART. 2. PETROLOGY AT GOOSE CREEK SHANNON. 47 



The feldspar of the altered zone is completely filled with a fine 

 flaky sericitic micaceous alteration product of high birefringence. 

 This extensive alteration masks the feldspar, making it indetermin- 

 able, although there is some evidence that it has been albitized in 

 the altered zone, as well as sericitized. Titanite occurs in euhedral 

 crystals along the crack and in irregular areas pseudomorphous after 

 iron ore in the altered rock. Near the outer border of this zone 

 replacement of titaniferous magnetite by titanite in all stages of 

 completeness may be observed. 



The principal reactions of the alteration have been the removal 

 of iron and titanium from the augite and magnetite, with the sub- 

 stitution of some lime. The titanium has recombined in the titanite 

 and has remained behind while the iron has apparently been removed 

 from the vicinity. The total amount of alkalies has doubtless been 

 increased by addition of soda to the feldspar in the form of the seri- 

 citic mineral, which is probably paragonite, and also perhaps as 

 albite. 



One diopside seam entirely like the one described and in normal 

 rock was seen, having a central layer a millimeter or two thick of 

 granular purplish axinite. Another diopside seam which cut an 

 aplite dike contained a central seam of axinite in the diopside and 

 widened occasionally with minute cavities which contained tufts of 

 fibrous hornblende with crystals of axinite and epidote or were lined 

 with axinite crystals and later filled with a white mineral, probably 

 apophyllite. These occurrences are more fully described below 

 under axinite. 



CHLORITE SEAMS ACCOMPANIED BY HORNBLENDIZATION OF THE NORMAL ROCK. 



Certain seams which resemble the diopside seams in having a deep 

 green to black median line have been found to have a central filhng 

 of chlorite accompanied by hornblendization of the augite of the adja- 

 cent normal diabase. A typical example of such seams is shown in 

 the illustrated specimen, plate 3, which may be described as follows: 



The central chlorite-filled crack is ^ mm. in average width and on 

 each side of this for a width of 1^ to 2 mm. the altered rock has a 

 vivid deep green color. Beyond this there is a border a millimeter 

 or so wide where the rock is whiter than normal. 



In thin section under the microscope the central crack is seen to 

 be filled with chlorite in aggregates of curved scales which yield shad- 

 owy extinction. This chlorite is intensely pleocliroic in tones of deep 

 blue-green parallel to the plates and pale brown in the direction per- 

 pendicular to the cleavage. The elongation of the traces of the 

 plates is Z; so, assuming that the acute bisectrix is perpendicular to 

 the basal cleavage, the mineral is negative. The chlorite merely fills 

 open spaces and does not replace any of the primary minerals. 



