\ 



174 



BULLETIN OF THE 



the fact that upon Marblehcad Neck the fclsitcs arc not all of tho 



same age. 



The stratified group which we have already described separates the 

 adjoining felsitcs into two distinct areas, each of which includes several 

 felsites of different periods of eruption. The felsite which is so inti- 

 mately associated with the ashes upon Breakdicart Hill occupies a con- 

 siderable portion of the area east of Main Street, and between Break-heart 

 and Little Castle Hills. A junction between the dark-colored felsite and 

 a pale pink felsite may be traced across the northern part of Little 

 Castle Hill, and southwest of this hill, about halfway to Main Street, 

 another distinct junction occurs along which there is \cvy well marked 

 banding in the light-colored felsite, showing that it is the younger. 

 Neither of those felsites is very porphyritic. 



Within the southern area, a short distance southeast of Long Pond, 

 Saugus, there is a small patch of a ree., chiefly non-porphyritic felsite. 

 The region immediately south of the pond, and to tho southwest as far 

 as the Boston and Maine Eailroad, is occupied by a felsite which is 

 generally very porphyritic. The ground-mass of this felsite varies 

 somewhat in color, but it is usually light pink or pale purple, and 

 besides being the most completely and uniformly porphyritic felsite of 

 this region, very frequently envelops fragments of older rocks. It might 

 properly be designated the porphyritic, pebble-bearing felsite, for these 

 two features are comparatively uniform throughout the wide area occu- 

 pied by this beautiful rock. The red non-porphyritic felsite, to which 

 we have already referred, forms a very clearly marked junction with the 

 porphyritic felsite about one fourth of a mile southeast of the eastern 

 end of Long Pond, and the porphyritic felsite, which is distinctly banded 

 along the lino of contact, penetrates the adjoining red felsite, and en- 

 velops many of its fragments. Among the enveloped fragments in the 

 porphyritic felsite are found a few of granite, — a fact which accords witli 

 those we have already discussed in showing that the gi\anitcs arc older 

 than tho felsites. The pebbles of the old felsite are numerous and widely 

 distributed in the porphyritic felsite, and the evidence is so complete 

 and well marked, that, as it seems to us, there can be no doubt that 

 the two felsites were extruded at different periods. 



About a third of a mile southeast of Swain's Pond in Melrose the 

 porphyritic felsite is cut by a dike of what appears to bo a third felsite, 

 which is non-porphyritic and of a gi'ay color. The dike has very irregular 

 junctions, varies in width from a foot to eighteen inches, and can be 

 traced across an exposiu'c for a distance of thirty feet. The youngest 



