1880.] | 359 (Diller. 
About a third of a mile north of Broadway station a well marked 
dike of felsite occurs in granite. The dike is five feet wide, with very 
distinct walls, and can readily be traced up the cliff more than twenty 
feet and across the country for a quarter of a mile. On Baker’s Hill, 
Malden, a large dike of non-porphyritic gray felsite occurs. The 
width of the dike is about one hundred feet and its strike N. 30° W. 
At the east end of Long Pond, in Saugus, there are several distinct 
dikes of felsite in the same granite, which at the west end of the pond, 
as already stated, contains well marked inclusions of the stratified 
group. North of Pine Hill in Medford there are numerous dikes of 
felsite in granite. One of the dikes is thirty feet wide, slightly por- 
phyritic and has very distinct walls. In fact all along the junction 
of the felsite and granite there are frequent thrusts of the felsite into 
the granite and inclusions of granite in the felsite. The junction is 
very irregular and sometimes difficult to trace, but where the rocks 
are clean I have not failed to find the line of an exposed contact. I 
have never seen an example of the passage of felsite into granite. 
The failure to find the junction in any case has been due to the 
absence of good exposures. | 
The numerous points of contact observed around almost all the 
granite areas, as well as the distinct dikes of felsite in the granite, 
show that the two rocks are distinct formations and do not pass into 
each other by an insensible transition. 
The felsites which occur in dikes are in their present position un- 
doubtedly eruptive. These clearly eruptive felsites occur, as I have 
already stated, at frequent intervals throughout the whole area occu- 
pied by the felsites north of Boston. They are exactly like and 
inseparably connected with the felsites of all the large areas and prove 
that all the felsites of that region are eruptive.? There are other 
important facts, to be noticed hereafter, that point to the same con- 
clusion. 
It must also follow that the felsites in their present position are 
younger than the granite.® 
t According to Mr. M. E. Wadsworth there are numerous dikes of felsite in the 
granite along the Swampscott coast. 
2 Mr. Crosby admits that ‘‘ exotic petrosilex and felsite undoubtedly exist ”’ in this 
region, but he does not believe that all the felsites north of Boston are eruptive. 
3 Mr. M. E. Wadsworth in his paper ‘‘On the Classification of Rocks”’ has 
already drawn the same conclusion concerning the granite and felsite of Marble- 
head Neck.—Bulletin of the Museum of Comparative Zoology, Cambridge, Mass. 
Vol. v., No. 13, page 282. 
