470 ©. H. Hitchcock— Helderberg Rocks in New Hampshire. 
to describe as carefully as possible the details of the distribution 
of the Helderberg rocks in Littleton and the surrounding towns. 
We must, however, first describe a small geological map of the 
Ammonoosuc region. 
The following are the groups of strata indicated upon the 
map, in the supposed order of their age: 1, Porphyritic gneiss; 
2, Bethlehem gneiss; 8, Gneiss; 4, Lisbon group; 5, Lyman 
group; 6, Clay slate; 7, Cods group; 8, Swift Water series; 9, 
Helderberg rocks. 
These designations represent distinct formations. In some 
cases a lithologica] name is used for convenience, chiefly that 
the attention of geologists may not be diverted by a strange 
terminology. It is not pretended that every layer of the por- 
phyritic gneiss or clay slate is uniformly and absolutely porphy- 
ritic or a distinct argillaceous slate; but that these terms repre- 
sent great thicknesses of associated strata, characterized by the 
predominance of the lithological varieties mentioned. Other 
kinds of rock are associated with them, the two series forming 
two great systems, formed in two distinct periods. The use of 
geographical names for the other groups of strata will prevent 
any geologist from misunderstanding what is meant to be sig- 
nified. The map assumes that this area is occupied by eight or 
nine entirely distinct systems of strata. 
Three new geographical designations appear on this map for 
the first time, viz., Lisbon, Lyman and Swift Water. The 
terms Bethlehem and Coéds have been defined in my New 
er Logan.* Recent papers by Credner, Macfarlane and Hunt 
have recalled to my mind a conversation with Prof. H. D. 
po : 
the Middle States. He then expressed the belief that the sem1- 
Azoic groups of Pennsylvania, etc., were equivalent to the 
rocks recently denominated Huronian by Logan. Inasmuch as 
his opinion concerning the same rocks in New England cannot 
stand, and the “ semi-Azoic” strata seem to be of the same age, 
tion, which was stated also in his Pennsylvania Report. It will 
: epee is a two-fold 
series. There is a third member which in my forthooe Se 
, i e 
correspond exactly wit 
* First and Second New Hampshire Reports. 
