MESOZOIC TIME — TRIASSIC AND JURASSIC. 803 



4. The lines of trap on the map are usually curved, with the convexities 

 to the west ; or they consist of a series of similar curves. Some are bow- 

 shaped with hooked ends. Saltonstall Ridge at S, on the east side of Salton- 

 stall Lake, near the Sound, is a marked example of the bow-shaped outcrop. 

 So also is the narrow line just east of it, and another broader and larger line 

 to the northeast, the Totoket Eidge, T T. The Mount Tom Ridge has an 

 eastward bend, hook-like, at its southern end, in the Meriden region, and 

 another long one at its northern end, constituting Mount Holyoke. The 

 distance between the two hooked ends is over 50 miles, so that it is a very 

 long bow. West Rock Ridge has a hook at its southern extremity, and a 

 series of curves in its course to the north ; but it terminates northward near 

 where the Mount Tom Ridge ends, as if a sequel to the latter in formation. 



These features are so general that they seem to indicate some compre- 

 hensive method of origin. 



5. The belts are for the most part approximately parallel to the axial 

 line of the area, or nearly north-by-east in course. But there are many 

 exceptions, especially in the southern part of the area. 



The large north-and-south outcropping belts of trap usually have bold 

 features over the landscapes. This prominence is owing to denudation since 

 the time of the eruption of the trap, for originally the trap was probably all 

 under the cover of the sandstone. The hard igneous rock generally makes 

 the summits of ridges. The slope of the ridge in the direction of the dip of 

 the sandstone (eastward in the Connecticut valley) is usually gradual, and 

 along it the trap disappears beneath overlying sandstone ; but in the opposite 

 direction, the ridge has a bold front of columnar trap resting on the sand- 

 stone. At the contact with the trap, in a north-and-south ridge, the sandstone 

 appears to be horizontal, because its dip is not northward or southward, but 

 eastward. Only in a transverse section of such a ridge should the underlying 

 sandstone show its true inclined position. These facts are illustrated in the 

 figures on page 302. The general features of the bold trap front are better 

 shown in the following view of West Rock ; but the part exposed to view is 

 an east-cmd-ivest section, so that here the dip of the sandstone is exhibited. 

 Below the bold columnar front of such ridges there is usually a talus of 

 broken blocks of trap ; the removal of this talus (for road making) has 

 exposed the sandstone to view. (The nearly horizontal line below the out- 

 cropping sandstone is the course of a road.) 



The Palisades along the Hudson are another good example of a trap ridge. The bold 

 front of the Palisades faces eastward, the dip of the sandstone being to the westward ; 

 and as the ridge has a northward course, the underlying sandstone, which makes about 

 half the height above the river's level, presents a nearly horizontal line beneath the trap. 



The east-and-west outcrops of trap are generally lines of simple tj-ap 

 dikes; that is, of trap within the fissure up which it flowed. On the contrary, 

 each north-and-south outcrop in almost all cases is that of an outjioio of trap 

 from a supply fissure, which is situated somewhere to the eastward. Examples 



