530 



Report of the State Geologist. 



recrystallization, and w hen this is the case ;i considerable amount <>f 

 untwinned feldspar is present. 



These basic gabbros vary in grain down to very finely granular varieties, 

 which are identical in character with the "shear-zone" rock from Avalanche 

 lake, described by Kemp.* .V regular series can be made out from rocks 

 w ith ophitic structure dow n to these finely granular varieties. It seems to 

 the writer that these shear-zone rocks must have been originally of the basic, 

 ophitic type, and then they are often found in this much granulated condition 

 because less resistant than the massive anorthositee to the forces acting 

 on the rock. Where such hands arc present they would represent lines of 

 weakness. But their different mineralogic composition is not regarded as 

 resulting from such action, but as largely a primary difference. 



These gabbros may he regarded as basic segregations from the anortho- 

 site magma, or they may represent a period of igneous activity subsequent to 

 the anorthosites. They certainly have a much wider distribution. So far as 

 the writer is aware, data are lacking at present by means of which either of 

 these propositions may be demonstrated at the expense of the other. Later 

 observations by both Professor Kemp and the writer, clearly show that these 

 ophitic gabbros are younger than the anorthosites. 



If the writer is correct in his belief that the diabases are of Pre-Cambrian 

 age, they must have followed the metamorphism of the rocks, as they are not 

 affected by it. 



Post-< >i:i>ovi< iax Disturbance. 



After the deposition of the Palaeozoic rocks, came the period of the 

 Green mountain uplift in Vermont, and its effects were felt on the western 

 side of lake Champlain. Though but a comparatively few miles west of the 

 district in western Vermont characterized by sharp folds and thrust faults, 

 the effects shown here are apparently limited to normal faulting, accompanied 

 by a very trivial amount of folding. The dip of the Palaeozoic rocks is but 

 slight, seldom leaching ten degrees, and more commonly not over five degrees, 

 and is prevailingly to the north. Higher dips do occur, but are invariably 

 local and in almost every observed instance can be demonstrated to be due to 

 proximity to a fault. Very low folds are often to be made out, but can 

 hardly be considered as constituting a prominent structural feature. Further- 

 more, there seems to be no system in their presence or arangement. They 

 trend and pitch in various directions. Faults however abound, ranging from 



♦American Journal ol Science, Vol. XL.IV.. pp. 109-111. 



