REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 125 



No. 7. Granitic-Hypersthene-Diabase (Norite). 



Rocks of this type occur at Nahant, but not elsewhere 

 in Essex County. The granitic-hypersthene-diabase, a 

 distinctive type, is the principal rock-mass of Little Na- 

 hant and at Nahant, east of the Maolis Garden, while on 

 Bass point this rock is in part an augite-hypersthene-nor- 

 ite. On Pea island and the ledge adjoining it at Nahant it 

 has the panidiomorphic structure of the dyke rocks, in- 

 terrupted by areas of porphyritic struktures of the older 

 effusive rocks. 



EFFUSIVE VOLCANIC ROCKS , PORPHYRITIC STRUCTURES, 



INCLUDING VOLCANIC FRAGMENT AL ROCKS, TUFFS, 



BRECCIA AXD AGGLOMERATE. 



No. 8. Rhyolites ; Quartz-Porphyry. 



Under this head are united all of the so-called felsites, 

 bandedfelsites, porphyry-felsites, breccia-felsites, agglom- 

 erate and altered or rnetamorphosed forms of this rock 

 mass. On Cat island and on Marblehead Neck, south 

 east of the lighthouse, are good exposures of the form 

 called agglonierate or rhyolitic-tufa containing sharp 

 edged fragments of volcanic glass embedded in ashy ma- 

 terials. Much of this glass has been altered to quartz 

 and the ashy material to an earthy chloritic mass, but it 

 has the same general character as specimens from Break- 

 heart hill in Saugus, described by Prof. J. S. Diller in 

 the Bulletin of the Museum of Comparative Zoölogy, 

 Vol. vii, No. 11, p. 168. The entire area covered by 

 these rocks is too small to attempt to indicate the various 

 forms assumed on a map of this scale, but it is intended 

 to prepare a map, in the near future, on a scale large 

 enough to bring out the principal features of all of the 

 varieties of this most interesting formation. Prof. George 



