F. L. Mason — Camptonite of Hawes and JRosenbusch. 229 



Art. XXXII. — A new locality of the Camptonite of Hawes 

 and Rosenbusch ; by Frank L. Nason. 



In Hitchcock's " Geology of New Hampshire," Part IY, 

 " Mineralogy and Lithology ;" also in this Journal, vol. cxvii, 

 p. 147, Dr. G. W. Hawes describes a " group of dissimilar eruptive 

 rocks in Campton, N. H." Dr. Hawes calls these dikes respec- 

 tively, diabase, olivine diabase, diorite and syenite. There are 

 five dikes in all : one each, of the first three and two of the 

 syenite. One of these dikes which Dr. Hawes has called a 

 diorite, Professor Rosenbusch, in his last edition of the 

 " Mikroskopische Physiographic der Massigen Gesteine," p. 

 333, has called Camptonite after the locality where it was first 

 observed. 



In the summer of 1885, the writer, while crossing the Green 

 Mountains along the line of the Rutland and Burlington R. R. 

 between Rutland and Bellows Falls, Yt., encountered a pecu- 

 liar rock in a cut near the station called Summit. This rock 

 was observed to occur in the form of a dike cutting across the 

 quartzite and the gneisses of the Green Mountains. The dike, 

 however, had a N. W. dip, strike N. E., S. W., while the country 

 rocks dipped S. E. The dike is about six feet in width. Near 

 its outer boundaries the contact with the rocks through which 

 it broke prevented the coarse crystallization which took place in 

 the center. It does not appear on the surface or either side of 

 the cut. Careful search was made for it but without success. 

 The reason is assumed to be that the dike rock is much more 

 susceptible to weathering than the country rock. The hope of 

 tracing the dike to some central chimney or throat was thus 

 abandoned. 



The macroscopic description given by Dr. Hawes of his dike 

 No. 2, answers almost perfectly for this dike, save that the 

 Summit dike is darker and richer colored, the porphyritic 

 crystals of hornblende are fresher and of a larger size than the 

 New Hampshire specimens which I have seen. The crystals of 

 hornblende also appear to be more numerous in the Summit dike 

 than in the other. It is also mottled near the contact with 

 large crystalline nodules of calcite and occasionally of feldspar. 

 Magnetite, menaccanite and pyrite also occur. 



Microscopically the rocks present no striking differences save 

 what a macroscopic study would suggest. In some of the 

 mottled spots of the Summit dike, however, are cavities filled 

 with true glass into which project well-developed lath- shaped 

 crystals of feldspar. 



Am. Jour. Sci.— Third Series, Vol. XXXVIII, No. 225.— Sept., 1889. 

 15 



