GEOLOGY OF THE NORTHERN ADIRONDACK REGION 275 



The rocks at present found in the district embrace only a frag- 

 mentary remnant of those formed at this early time, a great 

 thickness of other rocks haying been laid down above them and 

 later worn away. Twenty thousand to twenty-five thousand feet is 

 not an exaggerated estimate of this thickness. It is not meant to 

 imply that a uniform amount has been removed from the whole dis- 

 trict, in fact there is every reason to believe that the opposite is 

 true, that the region has been for much of its history a rugged 

 one, and that much greater removal has taken place from the 

 hights than from the depressions. While this accumulation of 

 the rocks which have since disappeared was in progress, the 

 region was in all probability below sea level and keeping pace 

 with the deposit by a slow subsidence. Not improbably a great 

 p£rt of the accumulated thickness consisted of igneous rocks. 



Great igneous intrusions. After the present surface rocks had 

 become deeply buried, they were invaded from beneath by a series 

 of great igneous intrusions, broken up into patches and no doubt 

 pushed upward to a considerable extent. At the time of the 

 appearance of these intrusions the previous rocks had become 

 profoundly changed in character, so that they were much in their 

 present condition. The district embracing Essex, southern Frank- 

 lin and northern Hamilton counties felt the full force of this 

 invasion, the larger part of the present surface rocks in that dis- 

 trict consisting of these igneous rocks, while away from it they 

 occur merely in patches. The present rocks cooled far beneath 

 the old surface and have been brought to the present one by wear 

 and removal of the overlying rocks. They represent abyssal, cooled 

 masses, whence no doubt much molten material ascended toward, 

 and not improbably to, the old surface. 



These rocks may be grouped into four great classes, anortho- 

 sites, syenites, granites and gabbros, all undoubtedly derived 

 from some great parent molten mass below by some process of 

 differentiation. The anorthosite intrusion was the first, the 

 bulkiest, and is the only one whose precise limits have so far been 

 worked out. It was followed by one of syenite, that by one of 

 granite, and the gabbro intrusion seems to have been latest of all. 



