REPORT ON THE GEOLOGY OF ESSEX CO., MASS. 119 



Boston Society of Natural History, in 1876, by Prof. W. 

 O. Crosby, and the rnaps accompanying the ninth annual 

 report the U. S. Geological Survey of a part of Cape Ann, 

 by Prof. N. S. Shaler, 1890, no other attempt, so far as I 

 am aware, has been made to map the bed rock of any 

 portion of the county. 



The atlas sheets of the State Topographical Survey, 

 based 011 the U. S. Topographical Survey, have been used 

 in the coustruction of the accompanying map and all plot- 

 ting of the various formations has been done upon the 

 separate sheets in the field, thus giving more accuracy to 

 the work than could otherwise have been possible. 



In connection with the map work, at the outset, it was 

 planned to obtain as nearly a complete collection as pos- 

 sible of the minerals and rocks of the county for exhibi- 

 tion in the cabinet of the Peabody Academy of Science. 

 With this in view persistent collecting has been done each 

 year since 1887. Several thousand specimens of the 

 minerals and rocks have thus been obtained from all parts 

 of the county and covering nearly every outcrop. As the 

 work proceeded it soon became evident that a more accu- 

 rate map of the bed rock was needed than had been made 

 by previons workers, necessitating a very careful study 

 of the contacts, strike, dip and trend of the rock-masses. 

 Ovving to the difficulty of obtaining fresh, unaltered spec- 

 imens of the outcropping rocks constant drilling and 

 blastinghave been required to procure good specimens un- 

 atfected by the weather. This, together with the care 

 needed to verify and correct the accounts of previons in- 

 vestigators, has made the work exceedingly slow and 

 laborious. 



Many days, and even months, have been devoted to the 

 study of rock specimens from one single Station, while, 

 later, reneAved field work was required to correlate evi- 



