THE HOLYOKE SHEET. 457 



entirely clear from au inspectiou of the figure, showing- how it surrounds 

 multitudes of the trap fragments and insinuates itself inti » all sorts of narrow 

 and tortuous cre\'ices. It can be seen where the muddy mass has flowed 

 into steam holes broken into on the surface of the trap, and there is a dis- 

 tinct fluidal structure of partly concentric lines in the mass, each bending 

 less deeply into the cavity than its forerunner. The limestone is in places 

 brecciated by the internal motion and explosions of the mass, its fragments 

 in part rounded by solution and recemented; it is homogeneous in every 

 part, and shows no marked effects at the immediate contact. 



If it is clear that these fragments of trap have been molded in the 

 oolitic mud, it is equally clear that this mud has been involved in the liquid 

 trap. Indeed, the thin sections were made from a point in the wall of the 

 great trap sheet exposed in the railroad cutting at least 10 feet below the 

 surface and wholly included in the continuous mass. That the trap frag- 

 ments were liquid when they came in contact with the limestone is shown by 

 the endomorphic effects jjroduced in the trap itself at the contact. There 

 is generally a thin film of pure and transparent glass in contact with the 

 limestone; then comes a border, 3 to 5™™ wide, which is dense black from 

 the amount of fine magnetite grains precipitated in a colorless gromid and 

 contains exceedingly minute feldspar needles. In the larger fragments the 

 feldspars gradually enlarge toward the center and the magnetite diminishes 

 until a normal trap results in which the larger generation of feldsj)ars 

 contains fine ramose glass inclusions, but in fragments less than 10""™ in 

 diameter the whole section is dense black. The small, round steam holes 

 are much more abundant in these borders than farther within the normal 

 trap. 



Streams of the small rounded grains of limestone can be seen penetrat- 

 ing the trap, running into it for several millimeters. The grains are in great 

 numbers, at first in contact with each other and without trace of intervening- 

 trap, and as the stream is followed inward the rounded grains separate and 

 float freely in the trap. They are distinguished from the secondary steam- 

 hole fillings of crystalline calcite by being of finely granular material, often 

 dusted with black trap grains. This black trap dust is abundant in places in 

 the larger liinestone masses and is an indication of the shattering of the hot 

 trap by the oolitic mud. The true steam pores are filled with diabantite or 

 calcite coarsely crystallized in transparent masses showing many twin lamintv. 



