374 E. O. Tlovey — Trim Ridges of the 



seem to be sphserocrystals or the result of rapid cooling. No 

 glass has been observed in these specimens, but the trap appears 

 to be noncrystalline, except as alteration has taken place. 



The macroscopical contact phenomena are well known, and 

 have been described many times. They are, the induration or 

 "baking" of the sandstone, the change of its color, and the 

 production of chlorite and zeolites in its seams and cavities by 

 the action of steam ; while, in a thick sheet, the trap from the 

 middle is much more coarsely crystalline than that from the 

 sides. Prof. Davis states that the indurating or metamorphic 

 effect of true dikes upon adjacent rocks is slight, and may be 

 taken as extending from the dike to a distance equal to one- 

 tenth of its thickness.* My own observations lead to the con- 

 clusion that no rule even approximately correct can be laid 

 down. The most accessible of the pronounced dikes of the 

 third western range are on the road from New Haven to East 

 Haven half a mile east of Tomlinson's drawbridge (at F, on the 

 map), and in the second and third railroad cuts east of Center 

 street, Fair Haven East. The walls of some of these dikes 

 show great variation in the amount of baking done to the dif- 

 ferent strata composing them. A layer of rock which has 

 been compacted and indurated to a distance of from one to six 

 feet from the trap will be overlain by shaly sandstone or 

 coarse sand which has been affected in this manner but a few 

 inches from the contact, and this in turn will be overlain by 

 indurated rock. A nine foot dike on the East Haven road 

 stands between walls of hard sandstone about 18 inches thick, 

 but the same dike is exposed under a barn 450 to the north 

 with its western wall composed of purplish black sandstone 

 which has been indurated to a high degree for more than eight 

 feet from the trap. The eastern wall has been removed, either 

 naturally or artificially. This variation in the relative amount 

 of baking of the different layers through which a dike has 

 passed may be explained by supposing that some strata con- 

 tained more water than others which were comparatively dry 

 at the time of the eruption of the trap. Wet sand or other 

 rock material being a better conductor than dry, the heat of 

 the dike would penetrate farther into the wet layers than into 

 the dry ones ; furthermore the hot water and steam thus pro- 

 duced would dissolve silica, which, on being deposited again, 

 would cement the particles of sand together. Dry heat not 

 greater than that which the trap probably had at the time of 

 its eruption would have no consolidating effect upon sand 

 composed, as the Connecticut Triassic is, of quartz, orthoclase 

 (and microcline) and mica. 



* This Journal, III, xxiv, 346, ]882. 



