248 TRANSACTIONS OF THE [may 18, 
ON THE EXISTENCE OF PRE-CAMBRIAN AND 
POST-ORDOVICIAN TRAP DIKES IN THE 
ADIRONDACKS. 
By H. P. CUsHING. 
CONTENTS. 
PAGE. 
Pn troduction Joc. oie. cues ccs ncieesecehes wate dees de ccc seeoeds mele ceies scale ene tae cveetaeeeee 248 
Present Rock: ClassificatlOn 24. cence dese sasetewecdos cess cestiseoncicee sme as sea aeeer 248 
Proposed Change in the ‘Classification <2. cv -c2.c,<- sce. «eas 0s cece eaten sahew eee 249 
THE IDIRES ee. wsacces sk occaccssoscsscctessscevs ollissses coskelems aticteecocsese deena mace 249 
IDESCHIPUION: o.cacenscciteoncscsoue sent “ee sseinicce oo oge Nese eeraecaekeeaceeaemeeneeees 249 
DISET BUGLON: side seats cas cade esoete none s.caclacee coe nbs Ce telun sales abe eer ceen eae 250 
Scarcity north of the Adirondacks! <7. .-.-+c.+-c+--ess-seoce eee sa cenenenene 251 
SUMMA ave -ciiocsssnscecesssestecscnderecedscscaeesescesseas toncecnecsecceccec emanate 292 
INTRODUCTION. 
As one commences field work in the eastern Adirondacks, the 
numerous dikes of the region early thrust themselves on the at- 
tention, and as they are studied and mapped certain facts regard- 
ing their distribution become obvious and demand explanation. 
These facts are more prominently brought out in Clinton county, 
N. Y., where the writer has been at work, than is the case in 
Essex county and in western Vermont, where the dikes have 
been so thoroughly studied by Prof. Kemp, though many of 
them have been noted and published by him. In the writer’s 
opinion they lead so irresistibly to the conclusion that there have 
been two periods of dike formation in the Adirondacks, quite 
widely separate in point of time, that it is deemed wise to pub- 
lish the evidence even though at present it falls short of absolute 
demonstration. 
PRESENT Rock CLASSIFICATION. 
Kemp has recently published the following scheme of classi- 
fication for the rocks of the eastern Adirondacks,* as the best 
that can be done with our present knowledge of the district. 
With the exception here discussed, the writer fully agrees with 
his generalization. 
The series is as follows, commencing at the bottom: 
I. “A gneiss series, usually laminated, but at times quite 
massive. The gneisses almost always contain quartz. The 
*J, F. Kemp, Rep. N. Y. State Geol.., 1893, Vol. I., p. 144. 
