92 B. K:. EMERSON — PLITMOSE DIABASE AND PALAGONITE 



Page 



Chemical analyses 112 



Composition of the glass and inclosing lava 115 



Theoretical explanation of the formation of the holyokeite and palagonite and 



their inclusions 117 



The general process 117 



Formation of the holyokeite dikes 117 



Formation of the glass with calcite spherulites 118 



Formation of lithophysae with sphserocrystals of ankerite and quartz 119 



The palagonite of Seljadalr 121 



KSsume" 124 



Discussion by Alfred R. Lane 1 25 



Explanation of plates 127 



Extent, Thickness, and general Structure of the Trap Sheet 



The Holyoke trap sheet begins south of Amherst and extends 80 miles 

 across Massachusetts and Connecticut to Long Island sound. It is about 

 300 feet thick in the latitude of Holyoke and has a low dip eastward, 

 resting conformably in the Triassic sandstones (see plate 24), and plainly 

 was a submarine flow.* It and its neighbors are composed for the most 

 part of a monotonous diabase ; but peculiar structures have been locally 

 produced by the introduction in various ways of the mud, sand, and 

 water of the sea bottom into the interior of the liquid and moving mass. 



Types of foreign Inclusions in the Diabase 

 me ride n type: blending of mud and lava at the base of the bed 



I have elsewhere described considerable areas where the trap, pro- 

 tected below by a thin solid crust, has flowed over and rested on the 

 muddy bottom, and then, the crust becoming ruptured and the liquid 

 lava coming in contact with the mud below, many steam explosions have 

 blown fragments of the under crust, with much mud, water, and filaments 

 and tears of the lava, up into the still liquid mass. In one case a pipe- 

 hole was blown clear through the sheet and a mud volcano formed on 

 its surface. By the blending of the cold mud and the liquid lava much 

 basic glass was formed, which differs much from the glass especially de- 

 scribed in the present paper (see page 103). By the above process minerals 

 like segerine-augite and structures simulating those of the crystalline 

 schists have been produced. These basal explosions are not of limited 



♦See description in geology of Old Hampshire county. Monograph xxix, U. S. Geol. Survey, 

 1898, p. 446. 



