HEAT. 



299 



Pine Rock is 35° to 40° from a vertical, and that of Mill Rock, 22°. East 

 Rock (in which the surface of trap is widened by a short westward outflow) 

 owes its subdivision into short, blunt parts (A,BB',CC'C",DD) to the same 

 cause ; the dike has a dip of 45°. The weak sandstone walls of these dikes 

 were at least 4000 feet in height, and a downfall of the unsupported wall was 

 a natural result. (D., 1891.) The same cause opened an escape fissure to 

 the north of Mill Rock, at D. 



267. 



Map of trap dikes, near New Haven, Conn. ; figures give heights above sea level. D. 



The rock of the outer portion of a dike, besides having the fineness of 

 texture and cracks due to rapid cooling, may be soft from alteration, or 

 may have a stratified appearance parallel to the walls, as in Figs. 268, 269 ; 

 or parallel fissures occupied by some infiltrating mineral ; and occasionally 

 they are vesicular. 



(b) Surficial streams. — The most extensive of nearly horizontal igneous 

 outflows — that of the Deccan, India — covers an 

 area of 200,000 square miles, and is of the age of the 

 Cretaceous and early Tertiary periods. It reaches 

 from the seacoast at Bombay to the railway station 

 at Nagpur, 519 miles. It was thickened by succes- 

 sive flows until 6000 feet thick near Bombay, 2500 



feet in Cutch, 2000 to 2500 feet at its southern 



Dikes with the columnar struc- 

 ture along the sides imper- 

 fect. D. 



limit ; to the northwest in Sind and to the southeast, 

 the thickness is only 100 to 200 feet. (Blanford.) 



Western North America, while remarkable for its 

 great volcanoes, is no less so for its non-volcanic rock-floods ; for these cover 

 nearly 100,000 square miles. The largest continuous region stretches from 



