A COMPOSITE DIKE 109 



four inches thick, and is a composite of basic diabase aphanite, coarse 

 plumose diabase, holyokeite, and quartz, and occurs near the center of the 

 abnormal band. The larger piece is deeply weathered, so that the two 

 lighter bands stand out a half inch on the surface. The smaller piece 

 shows a fresh fracture, and the central finer grained band is identical 

 with the left band in the piece below. If it had been pushed a half inch 

 more to the right the coincidence would have been more apparent. 

 Starting at the right of the large block, the first portion (a) is the fine- 

 grained diabase forming the ledge, presenting a weathered, uncharac- 

 teristic surface. Next, a narrow vein of quartz (6) projects strongly. 

 Third is a band of a fine-grained black diabase (c). Fourth, a broader 

 band of the coarse, long-plumose diabase (d) which appears also on the 

 upper fragment. Next is a band of the fine grained holyokeite (e), which 

 bends over from left to right in the larger block, and is deeply etched and 

 bleached, so that its outline does not show as clearly as it does through 

 the center of the upper block. Lastly, a narrow band of the " country 

 rock," the fine-grained diabase (a), appears in the lower rock and makes 

 the left hand half of the upper one. This has a coarser texture just 

 adjacent to the holyokeite (e) and grades into the common ground. 

 This is a light gray rock, more granular than ophitic in texture. 



The difficulty of explaining this occurrence arises from the fact that 

 apparently conclusive evidence is present to show that both the coarse 

 diabase and the holyokeite must have solidified first. The inch wide 

 holyokeite dike (e) was exposed for 7 feet, with constant characteristics, 

 and it would seem that the diabase must have solidified and cracked to 

 make a place for it. 



On the other hand, the coarse diabase has begun to crystallize on the 

 opposite surfaces of the holyokeite dike, and its great crystals bristle 

 out from it and diminish in size gradually as the coarse passes into the 

 fine diabase, and it would seem that the surface of the solid holyokeite 

 must have been there first. 



One may assume that the holyokeite was the result of a differentiation 

 which took place in an adjacent area, and that, having the composition 

 of albite, it would be highly viscid and would solidify at a higher tem- 

 perature than the basic and highly ferruginous diabase. 



It is perhaps possible, but only remotely probable, that the holyokeite 

 was carried by the violent motions as a broad, thin sheet of liquid ma- 

 terial (a schlieren) into the still liquid diabase while so viscid as not to 

 mingle with it, and then cooling first furnished the surface from which 

 the large pyroxene and feldspar blades shot out, forming the coarse dia- 

 base. The even thickness of the half-inch layer over at least two yards 

 militates against this. 



