92 



SCIENCE. 



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



belong to the same geological series, the 

 sedimentation involved more shales and 

 sandstones on the east and more limestone 

 on the vFest. To a certain degree the same 

 relations hold good for the Trenton series 

 to-day, its limestone being more massive on 

 the southwest of the crystallines and more 

 shaly on the eastern boundary. Neverthe- 

 less, for the pre-Cambrian formations, the 

 assured, fragmental sediments are still, as 

 emphasized above, comparatively thin and 

 scarce, and the inferences just stated re- 

 garding the gneisses vfIU arise. With a 

 view of throwing light on this question a 

 few typical sections will now be given in 

 some detail, and in the mind of the ob- 

 server or reader, the point of view should 

 always be maintained as to whether it is 

 possible to explain such relations by igneous 

 contacts, or whether we must not logically 

 refer them to a regular sedimentary suc- 

 cession. 



TYPICAL STRATIGEAPHICAL CROSS-SECTIONS. 



Catamount Mountain. — This is the most 

 northerly of the eastern outcrops. Although 

 the crystallines extend for miles beyond, 

 there are no more limestones. At the foot 

 of a steep mountain-side that looks away to 

 the southeast and that rises from twelve to 

 fifteen hundred feet above the valley, a 

 ledge of limestone has been well-exposed by 

 quarry operations. It is 20 feet thick and 

 150 feet long. It is a difficult matter to 

 convince oneself of the dip and strike, but 

 certainly the upper edge of the limestone 

 runs along quite regularly and considered 

 as a whole the rock seems to be a distinctly 

 bedded mass in other rocks. The banding 

 of the included minerals give a dip of from 

 45 to 60 degrees into the mountain. All 

 exposures of rock are concealed both above 

 and below the limestone so that its imme- 

 diate associates cannot be made out, but 

 out in the valley, in the road, a short dis- 

 tance to the south Gushing has noted an 



outcrop of a rusty friable gneiss consisting . 

 of nearly colorless monoclinic pyroxene and 

 microperthite. With these are sillimanite, 

 titanite, magnetite, pyrite and graphite. A 

 band of basic hornblende plagioclase gneiss 

 is also associated. These latter details I 

 quote from Gushing with whom, however, I 

 have been over the ground. In Wilming- 

 ton mountain to the southeast, I have found 

 further outcrops of graphitic rocks and of 

 hornblendic gneisses and pyroxenic aggre- 

 gates, such as are commonly associated with 

 the limestone. 



In passing up Gatamount mountain above 

 the ledge of limestone, no outcrops can be 

 found for a distance which involves some 

 hundreds of feet of cross-section, and then 

 a dark gneiss appears with parallel strike 

 and vertical dip. Under the microscope it 

 exhibits plagioclase, green augite, less brown 

 hornblende, garnet and magnetite, an as- 

 semblage that has strong affinities with 

 gabbros. Near the top of the mountain 

 this rock yields to a gneiss with abundant 

 quartz. I forbear to attempt to interpret 

 this poorly exposed succession at the 

 present, merely citing it as an illustration 

 of the relations met and of the difficulties 

 of the problem. 



TJie Western Spur of Whiteface. — From the 

 northern end of Lake Placid a wild and 

 narrow pass runs across a small divide, 

 separating the Ausable drainage from that 

 of the Saranac. At very nearly the crest 

 of the water-shed and in the foot of the steep 

 westerly spur of Whiteface mountain, a 

 double bed of limestone has been discovered. 

 The upper bench is 6 feet and the lower 12 

 with an interval of 25 feet occupied by 

 gneisses. Up the steep slope with a some- 

 what flattening dip, hornblendic gneisses 

 extend for 300 feet of section, then felds- 

 pathic gneisses for 300 feet more, until the 

 peculiar type of anorthosite of the White- 

 face massif appears. To the westward in 

 scattered exposures hornblendic gneisses 



