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very large collection of microscopic slides of the traps of the Mesozoic 

 sandstone in the vicinity of that town. 



Mr. Dana exhibited to me fragments of the traps, which when com- 

 pared with the fragments which I had brought with me seemed to be 

 identical lithologically so far as the eye, aided by a magnifying glass, 

 could determine. There were two varieties of this trap which had been 

 considered in my work essentially distinct, viz. : the doleritic and the 

 syenitic. Both these varieties are represented within a small area in the 

 immediate environs of Gettysburg, and even bear the appearance 

 of running together (to judge from a rough guess from the topo- 

 graphy). Now the finer-grained dolerite is of green color, and the speci- 

 mens from New England, and those I took with me, showed under the 

 microscope, and with the polarizer alone, the following mineral con- 

 stituents. Pyroxene (Augit), plagioclastic feldspar, magnitite (in fine 

 grains and irregular masses), and chrysolite. Mr. Hawes, of the Min- 

 eralogical laboratory, assures me that he has frequently found quartz in 

 these dolerites. 



The coarse-grained rock (both the specimen from Gettysburg and that 

 from Connecticut,) is gray and granular, consisting of black and white 

 crystals so mingled as to produce the familiar granite color to the eye. 

 In fact the rock from Gettysburg is called "Gettysburg Granite." It 

 was absolutely impossible to distinguish the fragments of this rock from 

 the localities apart, yet under the microscope and the single Nicol the 

 effect was very different. The Connecticut variety showed the same con- 

 stituents as the other traps — was in fact a coarse dolerite ; whereas 

 that from Gettys'iurg showed the characteristic dichroism of hornblende, 

 and also under a high magnifying power crystals of biotite. 



In the specimen which I took with me to New Haven, there were no 

 cleavage planes to absolutely settle the character of the supposed horn- 

 blende, bnt in others in my possession this was very marked and settles 

 definitely the question of the occurrence of syenite in the Mesozoic 

 sandstone. 



Mr. Dana warns me of a possible error in this conclusion, viz.: that the 

 mass from which I took my slides was only a bowlder — not in place. 

 This would be a very serious objection were it not for the absolu'e 

 identity of the rock in the immense masses of slab formed rock, from the 

 quarry which supplies the tombstones and the walls of our national ceme- 

 tery, as well as cubic roods of rock in Gulp's Hill, Great Round Top, 

 Granite Spur, and Devil's Den — localities which must ever remain 

 familiar to us as connected witli the history of one of the decisive battles 

 of the world . 



Besides this, as the Gettysburg locality lies miles south of the extreme 

 southern limit of the drift, there would seem to be no adequate theory to 

 account for such transportation. 



In order to set at rest this doubt and decide this question finally, fui-ther 

 sections will be made from rock without doubt in situ, and the resalts 

 coQimuaicated to the Society. 



