158 C. L. WHITTLE — METAMORPHIC CONGLOMERATE. 



the center having a pule blue to pale orange color in transmitted light, 

 although composed of but one individual, and is free from inclusions, 

 while the outer part is very dark and thickly sprinkled with grains of 

 gneissic quartz. 



►Still a fourth example (see plate 2, figure 2) has a dark core, with a still 

 darker rim, appearing as a network of bifurcating arms surrounding the 

 grains of the quartz and feldspar mosaic and on one side penetrating irreg- 

 ularly along cleavage lines into and enclosing parts of a clastic area of 

 plagioclase. Gaseous emanations or solutions carrying boracic acid are 

 well known to possess the power of attacking other minerals and produc- 

 ing new ones, and the action of the tourmaline-hearing agent, whatever it 

 may have been, seems to be another instance of the same phenomenon, 

 for the clastic plagioclase is attacked and the minute prisms of sericite 

 which had been developed in it along cleavage lines have been dissolved 

 by the growing mineral. Secondary glassy feldspars are included in the 

 tourmaline and reveal no evidence of having been attacked, but sericite 

 seems generally to have been absorbed. The clastic plagioclase is opti- 

 cally a unit with tourmaline ramifying through it ; hence the latter must 

 have eaten its way, excepting along cleavage lines, rather than to have 

 penetrated the feldspar along lines produced by its granulation under 

 pressure. Quartz does not seem to have been attacked, and one prism 

 of biotite has been built into the secondary mineral. 



Numerous cracks traverse the core, most of which abut abruptly 

 against the authogenic part, and, although fissures and irregular cracks 

 may traverse the outer portion, they are usually limited to the ends of 

 the prism and do not traverse the core. 



All the detrital tourmaline is macroscopically visible in the sections 

 studied ; nearly all are of an exceptional orange-yellow color, suggesting 

 a common source, and in size they vary from that of a pin-head up to an 

 eighth of an inch. 



It has been difficult to ascertain the source of the material forming this 

 conglomerate horizon in Vermont, and it is hoped that a microscopic 

 study of the terranes below may result in the discovery of this tourma- 

 line in situ and furnish a basis for further evidence. 



Two Periods of Disturbance indicated. — Two periods of dynamic action 

 are further indicated (see page 154) by the phenomena connected with 

 the tourmaline. Before the deposition of the allothegenic cores there 

 was a period of dynamic action, during which some of the detrital feld- 

 spar material was converted into an interlocking mass of quart/, biotite, 

 sericite and probably albite. Part of the quartz was comminuted, and 

 the fissures were produced in the tourmaline elastics. After this action 

 had ceased and the secondary tourmaline had been formed, a second 



