92 
regarded as branches from this dike? То actually trace the 
connection is impossible, on account of the marsh; but I have 
endeavored to show its probability by demonstrating more fully 
the identity of this melaphyr with the dike and also with the 
surface flow. To this end, characteristic specimens of mela- 
phyr from—I. the flow at the base of West Porphyrite Hill, 
II. the flow at the base of Great Hill, III. the irregular intru- 
sions in the basal conglomerate, and IV. the great dike, have 
been submitted to chemical analysis, through the kindness of 
Dr. T. M. Drown of the Massachusetts Institute of Technology. 
Four closely accordant determinations of the silica and of the 
iron oxide (Fe,O,) and alumina were made for each sample by 
the chemists of the class of 1892, with the following average 
results :— 
I II II IV 
SiO, 47.97 51.05 47.29 54.47 
Al,O,+ FeO; 30.50 30.16 30.20 25.25 
Although these results, especially for the silica, do not agree 
so well as could be desired, the differences should not excite 
surprise, when we consider that, since these rocks are all highly 
altered, and filled with quartz, epidote, and other secondary min- 
erals, it is well-nigh impossible to select average or normal 
specimens. Probably four specimens from the same mass 
(dike or flow) would show similar differences. Regarding, 
then, these different outcrops of melaphyr, of whatever form, 
as essentially identical in macroscopic, microscopic, and chemi- 
cal characters, we may reasonably suppose that they are con- 
temporaneous, and that the great dike is one of the vents at 
least, if not the only vent, through which the sheet of melaphyr 
overlying the basal conglomerate reached the surface. The 
fissure is straight and regular in the granite; but on passing 
into the then unconsolidated gravel of the basal conglomerate 
it naturally became very irregular, the melaphyr and gravel be- 
coming inextricably mingled, as observed on the eastern edge 
d 
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