GEOLOGY OF THE CONNECTICUT VALLEY DLSTRICT, 2/3 



Connecticut and Ammonoosuc rivers, and not including the whole of the 

 Helderberg area of Littleton. 



The following are the rock formations displayed in the Ammonoosuc 

 Gold field, arranged in the supposed order of their age: i. Laurentian, 

 consisting of the porphyritic and Bethlehem gneisses. 2. Atlantic gneiss, 

 represented by the Lake division. 3. Huronian, embracing the Lisbon 

 and Lyman groups and the auriferous conglomerate. 4. Cambrian clay 

 slate. 5. Coos grouix 6, Swift Water series. 7. Helderberg quartzites, 

 slates, and limestones. 



The general arrangement of these formations appears in Fig. 27. The 

 first two gneisses of Laurentian age border the field upon the north-east; 

 and the second is probably repeated in Haverhill, though not here sepa- 

 rated from the following division, which borders the field most of the 

 way upon the east side. A spur of this Lake gneiss divides the Coos 

 group for a couple of miles in the north-east part of Lisbon. The west- 

 ern part of the field is a basin of the Lisbon group holding the upper 

 Huronian, clay slate, and Helderberg, the Coos group being confined to 

 the region east of the Ammonoosuc. The clay slates occupy two lines 

 of outcrop, separated by the Lyman group. The more eastern one is 

 auriferous. The Coos group is isolated from all connection with its kind, 

 being separated from its northern continuation on account of the eleva- 

 tion of a range of Bethlehem gneiss. The Helderberg series appears to 

 follow the course of a mountainous ridge along the centre of the field, 

 lying principally in Littleton. The map was constructed primarily for 

 the sake of portraying the peculiarities of these rocks, — hence all the 

 section lines upon it are seen to cross this area. A sketch of the Hel- 

 derberg rocks of New Hampshire has been published by us in the Amer- 

 ican yoiirnal of Science and Arts, for May and June, 1874, in which Fig. 

 27 appeared ; and it is used now, with the approval of the editors of that 

 magazine. We are also able to present additional information concern- 

 ing these rocks, and the other formations arranged about them. 



I. Laurentian. 



Very little needs to be said concerning the Laurentian areas, as they 

 just touch the borders of the field, and have been amply discussed else- 

 where. The porphyritic gneiss area occurs at the corners of the towns 

 VOL. n. 35 



