56 NEW YORK STATE MUSEUM 



the amphibolite to be squeezed in between its detached portions. In 

 fact, in the outcrop in question, the relative capacity for yielding of 

 the two rocks is shown in the dragged schistosity of the garnet 

 gneiss against the steep amphibolite contact. Moreover, one of the 

 most characteristic and widespread features of this latter rock is its 

 ability to break sharply or to pull apart into blunt lenticles under 

 the same conditions in which the garnet gneiss has yielded by a 

 more ready flowage. Accordingly, where, as in the present in- 

 stance, the amphibolite is observed to surround or nearly surround 

 a' mass of highly plastic garnet gneiss, the latter's xenolithic origin 

 is rendered the more probable. On the other hand, should it be 

 urged that the pinched-off lenticular form, similar to that assumed 

 by isolated masses of amphibolite in garnet gneiss, is evidence that 

 the garnet gneiss is itself also of igneous origin, it may be noted that 

 among rocks which have suffered such extreme structural deforma- 

 tion, lenticular form is no criterion of intrusive origin, but only of 

 a relatively lower degree of plasticity. Under the conditions 

 imposed upon the Grenville sediments and associated igneous 

 rocks during their regional deformation, the competent formations 

 have fractured or pulled apart and the incompetent ones flowed in 

 between, giving rise to a kind of augen structure on a large scale. 

 This type of evidence, therefore, in the present case is inconclusive, 

 and the igneous origin of the garnet gneiss has not been suggested 

 by any other feature which is ,not equally ambiguous. , 



The second case of an intrusive amphibolite contact occurs on the 

 west slope of the hill about 1.4 miles south by west from the Pierre- 

 pont crossroads. The ledge is on the left bank of the tributary to 

 Van Rensselaer creek, at about 840 feet elevation. It is shown in 

 plate 9, upper figure, which looks very nearly in the direction of the 

 strike, S yz'' E. The lower part of the contact plane, as exposed, 

 dips south, the upper part, north. To be normal, this contact 

 ought to dip wholly to the north as do the others in the imme- 

 diate vicinity, and its failure to do so suggests the possibility of its 

 being part of an irregular intrusive boundary. The evidence, how- 

 ever, is not at all conclusive, for the same effect could conceivably be 

 attained through dynamic agency alone. In a nearby ledge some 

 tight crenulations in a pegm.atite veinlet pitch 20 degrees toward 

 the northwest. A more open fold pitching in this direction might 

 account for the attitude of the amphibolite in plate 9, upper figure, 

 but if folding alone were the cause, there should be a marked 

 surface deflection of the amphibolite to the right (S) not over 15 or 

 18 feet away, where the upper end of the axis outcrops. Such, 

 however, was not observed. The foliation of the garnet gneiss dips 



