No. 40] GEOLOGY OF SHEPAUG TUNNEL 29 



A smaller mass of the same diorite lies immediately south of 

 East Morris and extends north onto the southerly edge of this 

 map in the midst of the Hartland schist. It is about one mile 

 long and one-quarter of a mile wide. 



The Brookfield diorite is not a single simple intrusive but repre- 

 sents a number of different textural and mineralogical types. 

 The characteristic diorite from which the formation was named 

 is a green and white, mottled, medium-grained rock with a granitic 

 texture (Plates ii b and vii a). It is essentially a hornblende, 

 biotite, andesine or oligoclase diorite with variable but usually large 

 amounts of apatite and titanite, occasionally some diopside and 

 small amounts of quartz. There are two variants of this massive 

 type, one a light -colored quartz diorite composed of quartz, biotite 

 and andesine, the other a fine-grained, granular, porphyritic diorite 

 similar in composition to the first type described but with the 

 addition of scattered andesine phenocrysts from one-eighth to 

 one-quarter of an inch in length. There are recurrent masses of 

 fine-grained, schistose diorite or coarse diorite gneiss throughout 

 the tunnel that do not differ mineralogically from the granular 

 textured types. The foliation is due to the development of more 

 biotite, sometimes at the expense of the hornblende, with definite 

 orientation and with the consequent formation of bands richer in 

 hornblende and biotite, or in feldspar as the case may be ; and 

 to some straining and bending of the biotite and plagioclase and 

 granulation of the quartz when present. 



The strike of the banding in the foliated diorite parallels that 

 of the surrounding schists as a general rule but veers toward the 

 northwest both east and west of the northern angle in the tunnel. 

 The trend of the intruded porphyry dikes and the inclusions of 

 Hartland schist changes with the diorite and the dip swings to 

 the northeast or east. This does not bear any relation to the trend 

 of the diorite border since, before the Hartland schist is reached, 

 the strike swings back to the northwest and the dip to the west, 

 the normal dip and strike in the area, parallel with the foliation of 

 the schist and nearly parallel with the contact. 



So far as can be determined from the surface outcrops, this 

 change in strike and dip does not occur in the other formations 

 but is confined to the diorite and its inclusions. 



Dikes of gneissoid, quartz monzonite porphyry from a few 

 inches to twenty feet thick cut the diorite throughout its extent 

 and outcrop at various places, such as on the tip of Dempsey's 

 Point on the north shore of Bantam Lake. This rock is conspic- 

 uous because of white or pinkish phenocrysts of microcline from 

 one-quarter to more than three inches in length (Plate v a) set at 

 all angles in a groundmass of quartz, potash feldspar, green biotite 

 and andesine. Pyrite, apatite, and muscovite form the accessory 

 minerals. The groundmass of the dikes usually possesses a rough 

 gneissoid banding due to the segregation of the biotite into sepa- 



