No. 40] GEOLOGY OF SHEPAUG TUNNEL 25 



in order fully to appreciate the varying effect of igneous impreg- 

 nation and injection upon the rock. The sandy, micaceous, and 

 garnetiferous types with crystalloblastic texture far from any 

 intrusives are certainly to be explained by dynamic metamorphism 

 but the presence, at that time, of molten magmas underlying the 

 area must have had an effect. The change from a simple recrys- 

 tallized schist to a coarse injection gneiss that takes place in the 

 neighborhood of the larger dikes and stocks of granite, is the 

 effect of a cause that was doubtless at work to a lesser degree over 

 a much greater area. 



One-eighth of a mile south of the outfall on a new road that 

 skirts the valley into which the waters of the tunnel will flow, 

 there is an exposure of closely folded schist and quartzite cut by 

 pegmatite dikes in part earlier and in part later than the folding. 

 These quartzite beds are worthy of note because they show a 

 development of hornblende drawn out into pencils, small frag- 

 ments of apatite, large skeletal crystals of garnet that appear to 

 the naked eye as blurred, reddish brown spots on a white back- 

 ground ; biotite, zircon, and a little magnetite. These minerals 

 are best developed near the contact and, together with some granu- 

 lation of the quartz, give the rock a distinct foliation. These 

 minerals are due to the recrystallization of the impure parts of 

 the original sandstone near the contacts. There are a number of 

 narrow bands of similar rock in the tunnel. 



There is another type of alteration in the same rock in which 

 hornblende and garnet are developed and the hornblende forms 

 unorientated laths. This type is associated with the small peg- 

 matite dikes that cut the quartzite and schist and is believed to be 

 due to contact metamorphism by solutions emanating from the 

 pegmatite. Some of the "pencils" of hornblende mentioned under 

 the first type of alteration seem to be cut off abruptly or eaten into 

 by quartz later than the recrystallization. Veins or irregular 

 patches of crystalline calcite are common in certain parts of the 

 quartzite. 



Two narrow calcareous beds in the short tunnel under the 

 southern end of Guernsey Ridge represent a third type of alter- 

 ation. They are composed of granular quartz, hornblende in 

 unorientated blades, garnet, zoisite, titanite, muscovite, apatite, 

 calcite, and considerable plagioclase with an index higher than 

 that of quartz. None of these minerals are strained or crushed 

 and the rock appears to be a mixed type — an impure limestone 

 layer recrystallized and then penetrated by quartz and plagio- 

 clase. There are no large dikes in the neighborhood so that the 

 solutions must have traveled far and performed a selective 

 replacement. 



The part that such impregnations and replacements have played 

 in the total metamorphism of the area here under discussion is 

 relatively slight, but it is believed that the neighboring areas, 



