HOLYOKEITE, A PURELY FELDSPATHIC DIABASE 5 1 1 



This ignores the water which might have been calculated, 

 perhaps, as kaolin, since there is no visible chloritic mineral or 

 zeolite. It ignores also a third of a per cent, of CaO, which now 

 forms calcite, and which with the rest of the calcium was present 

 in the original rock, perhaps for the most part, as anorthite ; 

 that combined with the albite formed several intermediate varie- 

 ties of plagioclase. 



This calcium may have been present in part as a sahlite. In 

 the latter case it might have been called a sahlite-diabase, but 

 in a sense very different from that in which the word has been 

 used by Tornebohm for a diabase in which the sahlite is quite 

 subordinate to the abundant augite. The rock in which the frag- 

 ments are embedded is, however, so calcareous that some part of 

 the calcite may have been introduced into the amygdaloidal cavi- 

 ties from without. 



The leucophyr of Gumbel is, as compared with diabase, "a 

 remarkably light-colored rock with saussuritic feldspar, pale 

 green augite (without hornblende and rarely with red-brown 

 augite), with a chloritic constituent in large quantity and tabular 

 ilmenite." This seems to me quite plainly an altered diabase, 

 and different from this non-ferruginous rock. 



The only similar rock that has been described from the 

 American Trias is the acid dyke discovered by Mr. E. O. Hovey 

 in the new cut on the Shore Line Railroad, in the eastern part 

 of New Haven, provisionally referred by him to keratophyr. It 

 is distinctly different from the type described here. 1 It is brick- 

 red like the Cheapside trap, or grayish like the second variety 

 described above, which accompanies the white trap. 



It is largely feldspathic and the ferro-magnesian minerals are 

 absent, but chlorite is present in considerable abundance, and the 

 fields of chlorite and calcite " for the most part have definite 

 outlines which strongly suggest the original presence of pheno- 

 crysts of pyroxene (augite) in the rock." Mr. Hovey remarks 

 that spherocrystalline structure was observed in some places. 

 This may have been the structure that I have interpreted above 



1 Am. Jour. Set., ser. 4, Vol. Ill, p. 287, 1897. 



