276 



NEW YORK STATE MUSEUM 



All three have a much wider range and a more patchy distribution 

 than the anorthosite. Their precise importance is however un- 

 certain, since there are certainly granites and gabbros, and likely 

 syenites also, of more than one age in the region, though quite 

 similar to one another, so much so that no criteria have yet been 

 developed for their discrimination. This difficulty has not to be 

 met in the case of the anorthosite. 



At some time after the cooling of these great intrusions the 

 whole district was subjected to great compression, as a result of 

 which all the existing rocks were profoundly changed in character, 

 the intrusives as well as the older rocks. But, since the igneous 

 rocks did not experience the earlier compressions, as did the 

 others, and since these must have been profoundly affected by the 

 heat and pressure of the intrusions themselves, the intrusives are 

 less completely altered than are the older rocks, and frequently 

 retain traces of their original structures and textures, often in 

 considerable amount, so that usually their origin and nature are 

 not open to question. This is specially true of the anorthosites, 

 which are mostly very coarse grained, porphyritic rocks, but it is 

 frequently true of the others also. The rocks were more or less 

 mashed and recrystallized, and rendered gneissoid in greater or 

 less degree, the same rock varying much from place to place in 

 these regards. It is the more gneissoid phases which are most 







difficult to distinguish from some of the older rocks. 



The character of the changes produced indicates that the rocks 

 were under great load during the compression, or, in other words, 

 were deeply buried beneath overlying rocks. 



Great Precambric erosion. Precambric time was very long, 

 not improbably comprising as much as or more than one half of 

 the earth's geologic history. During most or all of the later part 

 of this vast time interval the region was a land area and under- 

 going wear. The overlying great thickness of rock under which 

 the present surface rocks lay buried at the time of compression, 

 was removed in Precambric time in greater part. Quite likely 

 the time of elevation into a land area coincided with the time of 

 compression, the two being effects from the same cause. The great 



