ART. 2. PETEOLOGY AT GOOSE CEEEK SHANNON. 37 



for the phenomena, in theh' relation to the phenomena observed in 

 the diabase at the Goose Creek quarry. 



The quarry is located practically at the base of a large intrusive 

 tabular mass of diabase, probably a sill or a very flat dike, some 

 hundreds of meters thick. Such a mass should be expected to dif- 

 ferentiate with the sinking of crystals, provided that cooling did not 

 take place too quickly. At the base, however, the heavier and more 

 basic minerals should be concentrated by this process, giving gab- 

 broic rocks, while higher in the sill the more acid differentiates should 

 be found, particularly at or near the roof. There is, however, no 

 evidence of any banded structure in the diabase, nor any increased 

 basicity toward the bottom, and the rock to the very bottom of the 

 mass, exclusive of a possible chilled border phase, seems to be repre- 

 sentative of the average of the mass. It is a diabase not in any 

 wise different from the undifferentiated basaltic rocks of the Newark 

 series, as shown by the comparison of analyses above. Although 

 observed occasionally, micropegmatitic interstices are not a conspicu- 

 ous feature of the rock, and the plagioclase is almost free from zon- 

 ing. It is concluded from these facts that this diabase mass as a 

 whole cooled too quickly to give the differentiation effects obtained 

 upon slow cooling of diabasic magma and consequently crystallized 

 simply as a mixture of basic pyroxene and plagioclase. 



The differentiates, ranging from rock only slightly different from 

 the normal diabase in composition to pure albite-quartz rock, are 

 another matter. These occur near the bottom of the sill instead of 

 at the top so that they can not have originated by the sinking of crys- 

 tals, and they occur as numerous more or less insolated bodies of 

 small size.^^ These likewise can not be explained by the syntexis 

 or solution of engulfed blocks of the intruded rock unless they are 

 local syntetic bodies of material which has risen from the floor of the 

 sill through the diabase or blocks from the roof which have sunk 

 through the magma in the chamber to the bottom, there to be dis- 

 solved. Neither of these possibilities has any concrete support and 

 the latter possibility seems precluded, as suggested by Lewis, by 

 the fact that blocks of shale or limestone would float on the molten 

 basalt. It is possible that the solution of the shale or limestone in 

 the magma would yield the effects obtained but there is no evidence 

 to support such a contention. Such an explanation of the origin of 

 the granophyres of the Gowganda Lake region, first favored by Bo wen, 

 as quoted above, has since been abandoned by him:^^ 



Somewhat similar sills in the Gowganda Lake district of Ontario, described 

 by the writer, have essentially the same relations. In the original paper it was 



'8 It is of course possible that other similar digcrentiates occur near the top of the diabase sill but no 

 exposures favorable for study are available. 

 IS Norman L. Bowen. Later Stages of Evolution, etc. Journ. Geol., vol. 2, appendi.\, p .49, 1915. 



