GENERAL STRUCTURE AND CORRELATION. 33 



in its cement. We would find the limestone of the western column repre- 

 sented only by more or less calcareous material in the fine sediments of 

 the corresponding part of the eastern column, and by a rather abrupt 

 lateral transition through flaggy limestones and marls, containing more 

 quartz sand at the bottom and more clay at the top. Above this horizon 

 we would find the tine sediments alike common to both columns and 

 extending far both to the east and the west. 



Analyzing the different horizons we find along the west side of Hoosac 

 mountain different conditions of sedimentation affecting the horizons of both 

 the grit and the limestone. To the east the grit becomes a conglomerate 

 abounding in granitic pebbles and in detrital feldspar. To the east also the 

 limestone passes into shoreward argillaceous sediments. Higher up we find 

 in the uniformly widespread fine sediments the evidence of changed condi- 

 tions, which through a long period excluded to a great extent the formation 

 of limesti mes over the whole region. 



Such in a general way was the differentiated character of the rocks upon 

 which the processes of metamorphism acted. These processes resulted in 

 changing the quartz sandstone of the Cambrian grit into a quartzite, and the 

 shoreward feldspathic sandstone into a highly crystalline gneiss. The Cam- 

 bro-Silurian limestone, the limestone proper, was changed to crystalline 

 limestone; its sin. reward transitions into more or less calcareous gneiss and 

 its more eastward calcareous shales into a garnetiferous variety of the albitic 

 schist, into which the whole column of Cambro-Silurian fine sediments above 

 the lower Cambrian grit has been changed. In the finer sediments, the 

 uniform character above the horizon of the limestone resulted in a uniform 

 change into a mica-schist characterized by the general presence of albite in 

 macroscopic or microscopic crystals. 



We do not yet know to what depth these rocks were buried. They 

 have in themselves an aggregate thickness of 5,000 feet or more. Certainly 

 if they were covered by the great thickness of material represented in the 

 schists between Hoosac mountain and the Connecticut river, they were 

 buried to a point of load and temperature sufficient to satisfy these condi- 

 tions of metamorphism. 



Throughout the whole region all the rocks above the pre-Cambrian 

 MOM xxm 3 



