THE INCLUSIONS 349 



digenous rock series ; hence quite likely their thickness at the knob is con- 

 siderable, some hundreds of feet. In considering the manner of over- 

 thrusting of such weak shales, it seems reasonable if not obligatory to 

 suppose that the thrust was transmitted by some strong, competent rock 

 stratum on top of which the shales were carried westward. This stratum 

 might carry as far west as the shales, or not quite so far, a jumbled mass 

 of shales extending somewhat beyond it. The Bald Mountain limestone 

 is the first competent formation of this sort beneath the Normanskill 

 shales in the overthrust section, and it is therefore not unreasonable to 

 suppose that it may be present underground at the knob. If so it would 

 be nearest the surface of any of the limestone or dolomite formations of 

 the region, with soft shales above it, and also below it if it rests on the 

 Canajoharie, as is probably the case. Hence fragments of it could be 

 easily carried upward in the lava. The lack of shale inclusions is not 

 thus explained, nor the lack of inclusions from the more deeply buried 

 formations, such as the Little Falls dolomite and Potsdam sandstone. If 

 the drill should show that the Bald Mountain limestone was present at no 

 great depth below ground at the knob, or if, on the other hand, it should 

 show that it was not present at all, nor any other similar limestone for- 

 mation, we should have more definite data for deciding whether the knob 

 was exotic or not ; but the drill has not been at work in the vicinity. 

 With our present knowledge, it seems to us the inclusions are most simply 

 explained on the view that the knob is an overthrust fragment of a sur- 

 face flow poured out on limestone, though it is an open question whether 

 under such conditions the limestone would have been likely to undergo as 

 much corrosion as it has experienced. 



Age of the igneous Bock 



The question of the age of the lava is obviously connected with the 

 question as to whether it is in situ or is not. If in place, it must be 

 younger than the Ordovician shales which it cuts. If not in place, this 

 is not necessarily true, though it is probably so. There exist in New 

 York two groups of igneous rocks which fulfill this age requirement — 

 the basic dikes of the Champlain Valley and of central New York and 

 the (raps of the Newark series. When Woodworth and Oushing dis- 

 cussed this matter at the time of the original description of the knob, a 

 dozen wars ago, they independently suggested a Newark age as most 

 probable. The rock is much more like the Newark traps than it is like 

 the basaltic dikes of the Champlain Valley or (he peridot iles of central 

 New York, and a reference to a known group of igneous rocks seemed 

 to us a more probable one than the alternative of considering this as the 

 single representative known of igneous action of other date in the vicinity. 



