July 20, 1900.] 



SCIENCE. 



97 



form synclines, piBched into underlying 

 gneissoid rocks. On the contrary they 

 seem to constitute low dipping faulted 

 monoclines. 



All the sediments are thoroughly recrys- 

 tallized and metamorphosed and the asso- 

 ciated igneous rocks are plutonic or deep- 

 seated types. Both these facts indicate 

 their former burial at very considerable 

 depths, and the subseqvient removal of 

 some thousands of feet by erosion. The 

 next later rocks, of whose geological age 

 we are assured, are the Potsdam sandstones, 

 which lie on the old crystallines with dips 

 seldom if ever more than ten degrees and 

 which are not seriously metamorphosed. 

 The greatly metamorphosed sediments 

 are certainly pre-Potsdam and the large 

 tectonic relations of the Georgian strata in 

 Vermont to the Potsdam and the crystal- 

 lines preclude our considering the latter as 

 of possible early Cambrian age. We are 

 forced to conclude therefore that they are 

 pre-Cambrian, and from the comparatively 

 unmetamorphosed condition of the Cam- 

 brian beds, we infer that the pre-Cambrian 

 strata suffered their metamorphism in pre- 

 Cambrian time. They may be taxonomic 

 equivalents of the Huronian, but we have 

 no good grounds for correlation. 



The evidence regarding the Cambrian as 

 interpreted by Walcott in the Champlain 

 vallej'-, leads us to believe that the Cam- 

 brian sediments encroached from the east- 

 ward upon the area of the crystallines. The 

 Georgian is only found in Vermont. The 

 Potsdam alone appears on the western side 

 of Lake Champlain. It was not therefore 

 any load of Paleozoic sediments, which 

 rendered possible the deep-seated meta- 

 morphism of the pre-Cambrian sediments 

 and the plutonic textures of the intrusions, 

 but a load of pre-Cambrian rocks which 

 have since disappeared. What those rocks 

 were is an interesting subject of speculation. 



They may have been sediments, whose 



disappearance leaves us with a lost inter- 

 val. If so there is a gap in the records, 

 which would be more comprehensible if we 

 had better evidence of tight folds in our 

 pre-Cambrian sediments and not the com- 

 paratively iiat beds of limestone so often 

 seen. 



They may have been fragmental eject- 

 ments and vast surface flows of lava from 

 centers of eruption whose deep-seated roots 

 alone remain to us in the anorthosites, gab- 

 bros and syenitic rocks and whose materials 

 piled up in the not unreasonable thick- 

 nesses of some thousands of feet, have been 

 in time removed to contribute to the Cam- 

 brian or still earlier but undiscovered strata. 

 Certainly the period of erosion was long and 

 the results pronounced. 



Bearing these consideration in mind, 

 sometimes while seated upon a lofty peak 

 of the mountains and while reflecting on the 

 scene spread out in every direction, I have 

 allowed my fancy free play and have pic- 

 tured again the cones and vents that prob- 

 ably made of the Adirondacks a volcanic 

 center comparable with Lake Superior. 

 Beginning with eruptions of medium com- 

 position, as we know from the oldest igneous 

 rocks now present they passed to more 

 acidic types and closed with the basic gab- 

 bros. The fires seem then to have cooled 

 and long erosion ensued. 



Meantime beneath the piles of igneous 

 rock, metamorphism from the hot intrusions 

 and from the general rise of the isogeo- 

 therms went steadily forward, and the 

 ancient sediments, whether calcareous or 

 clastic, were changed over to marbles, 

 quartzites and gneisses. Their carbonace- 

 ous matter became destructively distilled 

 and penetrated every available crevice. In 

 time it was changed to graphite. It even 

 wandered over to the neighboring, partly 

 cooled, igneous rocks and took part in the 

 formation of the pegmatites. 



Gradually the early Cambrian sea crept 



