166 J. D. Dana — Additional Observations on the 



an inch thick by an intervening mass of sandstone ; hut this is 

 only an irregularity of outflow due to the way the sandstone 

 has of breaking, like that in the Mill Rock dike described and 

 figured on page SS of the paper above mentioned. 

 This branch dike has several points of interest. 



1. It affords new proof that the West Rock outflow took 

 place under a heavy cover of sandstone and therefore was 

 laccolithic. This conclusion was sustained, in my former paper 

 (p. 103), on the ground (1) that the outflow continued to be an 

 ascending one for the 500 yards of its westward flow — which 

 could not have been true unless it were under a resisting cover ; 

 and (2) the outflow retains a thickness of 250 feet quite to its 

 extreme western limit, which it could not have done if it had 

 been a subaerial, or, using a much needed new word, a sur- 

 Jicial, flow.* 



But (3) under laccolithic conditions, the opening of a fissure 

 in the underlying sandstone for a discharge from the lower 

 part of the mass of. trap or else from the dike, would be a 

 natural result. The liquid trap that was being forced up and 

 onward under the repressing sandstone formation, might readily 

 have made the fracture, especially when, after the lava had 

 attained nearly its full thickness, a new start in its movement 

 was given. Had it been a surficial stream it would have had 

 no fracturing power. 



2. Other points of interest in the dike are connected with 

 its constitution. (1) While the rock of West Rock is a com- 

 pact, rather coarsely crystalline doleryte, and has a light blue- 

 gray color owing to the abundance of labradorite with the 

 pyroxene, and is darker and finer but still gray at its junction 

 with the sandstone, the trap of the dike is black, without lus- 

 ter, and aphanitic. Moreover, (2) it is amygdaloidal or vesicu- 

 lar ; some of the cavities are over an inch in diameter and 

 contain a lining of quartz crystals with a filling often of 

 laumontite, while others are minute and filled with quartz. 

 The rock is besides irregularly jointed, and much rifted and 

 deeply altered along the rifts by weathering. 



Examined microscopically in thin slices, the rock is found 

 to have other peculiarities. (3) Magnetite is unusually abund- 

 ant in very minute grains, much more so than in the trap of 

 West Rock. (4) The labradorite of the rock is in crystals of 

 the usual form, but they are extremely small. (5) The gray 

 pyroxenic material about the labradorite is also in minute 

 grains and seldom shows color in polarized light. Further, (6) 

 greenish olivine is present in crystals and groups of crystals — 

 a mineral not yet observed in the West Rock trap.f 



* The word superficial is too various in its significations for the place. Sur- 

 ficial is like surface in having its prefix the French abbreviation sur in place of super. 

 f A full petrological description of the rock will be published later by another. 



