THE STRATIFIED ROCKS OF ESSEX COUNTY. 



BY JOHN H. SEARS. 



THE term "stratified rock" is applied to different rock 

 formations in which stratification is the only common char- 

 acter, and although the syenites, diorite, felsites and some 

 of the so-called breccias show stratification in part, there is 

 little difficulty in separating them from the groups con- 

 sisting chiefly of limestones, quartzites and argillaceous 

 rocks of which this paper treats. 



Nearly one-half of the bed rock of Essex County is dis- 

 tinctly stratified, and by means of our knowledge of these 

 groups the geologic age of all the other rock masses may 

 be approximated. 



When we consider the frequent faulting of the rock 

 formations and the great area which is covered by drift 

 sand, gravel, clay and till, leaving only occasional outcrops 

 for examination, the difficulty of preparing a correct map 

 of the underlying rocks of the county may readily be seen. 



The stratified rocks of the county which are here to be 

 considered are divided into several groups. The principal 

 ones are the limestones, argillites, quartzites, and shales 

 of detrital origin, and the schists, amphibole and granitic 

 gneisses of doubtful derivation ; the whole to be classed 

 as more or less metamorphic, a condition clearly revealed 

 by the microscope when these rocks are studied in thin 

 section. The effects of metarnorphisrn on rocks are con- 

 solidation, loss of material by chemical solvents, change of 



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