SCIENCE. 



[N. S. Vol. XII. No. 290. 



away they again appeared on a mountain 

 side with very nearly the same strike and 

 dip and there is no doubt that a very 

 considerable thickness is present. Gabbros 

 in one direction and anorthosites in another 

 cut them out, and on the strike they were 

 traced into exposures which contained lime- 

 stones. Graphite was abundant both in 

 limestones and gneisses. 



In many other localities these same rocks 

 have been met but mostly as isolated ex- 

 posures in the midst of the heavy forest 

 growth and too few in number to enable us 

 to work out their thickness or their accu- 

 rate relationships, but there is no doubt that 

 they represent sediments that must have 

 been originally of the nature of sandy shales, 

 which at times had more richly calcareous 

 layers and which, in this way, yielded the 

 variable metamorphic results, now acces- 

 sible to us. As a rule the dips of these 

 gneisses are low, although high dips are met. 



Besides the gneisses just described, which 

 exhibit the marked regularity in their band- 

 ing there are others that are more massive 

 and uniform, and yet that from their gen- 

 eral relations and associations give strong 

 evidence of belonging in the sedimentary 

 series with the limestones. They are almost 

 always rusty on their outcrops as distin- 

 guished from the certain eruptives and 

 whenever this character is observed we 

 commonly look with success for the near 

 presence of limestones. Although appar- 

 ently quite basic the microscope reveals 

 in most cases quartz and microperthite as 

 the light-colored minerals in the midst of 

 the prevailing hornblende and less augite. 

 Plagioclase is not lacking, but is decidedly 

 subordinate. Graphite has been occasion- 

 ally detected in them. 



These rocks have proved exceedingly 

 puzzling members to deal with in the field, 

 because one would be inclined at first sight 

 and from microscopic examination to regard 

 them as gneissoid gabbros or diorites, but 



the microscope gives the results just speci- 

 fied and the structural relations which will 

 be shortly taken up lead to the conclusion 

 that they are altered sediments, and that 

 they probably represent large and fairly 

 uniform bodies of shale. 



Professor Gushing has noted in the eastern 

 part of Franklin county considerable out- 

 crops of a very coarsely crystalline and 

 slightly rusty rock, which I have likewise 

 had the privilege of studying in the field 

 with him. It consists of almost nothing 

 else than lenticles of quartz, half an inch or 

 more wide, an eighth or more thick, and an 

 iuch or two long, which are set in a matrix 

 of microperthite. Practically no dark sili- 

 cates appear. I have also occasionally ob- 

 served the same rock further south and I 

 do not know how to account for it other- 

 wise than as a recrystallized and squeezed 

 conglomerate, whose pebbles have been 

 stretched and rolled out to the lenticles and 

 whose interstitial filling has yielded the 

 microperthite. If this view be correct, we 

 have all the ordinary members of a sedi- 

 mentary series represented among these 

 metamorphic rocks and a much more prob- 

 able association for an important and ex- 

 tended member of the geological column, 

 than would any one or two of the above 

 cited members be alone. It is quite possi- 

 ble that others of the more massive gneisses 

 are altered sediments rather than sheared 

 eruptives, but in the absence of positive 

 proofs I hesitate to take even a tentative 

 position regarding them, although I am free 

 to admit that beginning with prepossessions 

 in favor of the igneous origin of many of the 

 gneisses, I have become more and more con- 

 vinced that altered sediments play a very 

 prominent role. 



GENERAL DISTRIBUTION OF THE METAMOR- 

 PHOSED SEDIMENTS. 



The Northwest. — The crystalline lime- 

 stones furnish the most widely distrib- 

 uted, indubitable form of pre-Cambrian 



