SUPPLEMENTARY REPORT 



ON THE MINERALOGY AND GEOLOGY 



OF ESSEX COUNTY. 



BY JOHN H. SEARS, 

 Curator of Mineralogy and Geology, Peabody Academy of Science. 



During the seasons of 1894, '95 and '96, the principal 

 work in the geology of the county has been the study of the 

 quartzite and interstratitied beds in Lynnfield Centre and 

 North Saugus and carrying the work southwest into Mid- 

 dlesex County in the towns of Reading, Wakefield, etc. 

 The quartzite beds of North Saugus are, without doubt, 

 parts of the basal Lower Cambrian and the knowledge of 

 this formation has been extended southeast on both sides 

 of the head waters of Penny l)rook to Saugus river. The 

 quartzite and inters tratified slates and coarse conglomer- 

 ates of Lynnfield Centre are the basal members of the 

 Carboniferous rocks, a continuance of the blocked area, 

 No. 19 on the map, of Topsfield and Middleton. This 

 work makes several changes necessary in the preliminary 

 map published in 1893. As an evident mistake was made 

 in the mapping of the contact of the hornblende-granite 

 and the diorite areas of Marblehead and Swampscott, 

 these have been more carefully worked out and remapped. 

 Another correction made necessary is the separation of 

 the hornblende-granites from the hornl)lende-biotite- 

 granitite. This was not done when the map was published 



ESSEX I>'ST. BULLETIN, VOL XXVII 11* (109) 



