CLASSIFICATION OF THE NEW HAMPSHIRE FORMATIONS. 66 1 



sometimes have been inverted, — that is, moved more than to the perpen- 

 dicular position. 4. Vertical strata may be resolved into compressed 

 folds, — as many as the necessities of correlation seem to require. 5. 

 Where there is an ascending series of groups, any one of the number 

 may show several flexures. When their whole number is not known, the 

 band may seem to possess an anticlinal structure instead of both the 

 ridge and basin shape, or a fault may have cut away one of them. More 

 than one section is often required to determine the structure of a- partic- 

 ular band. 6. We have cases of what the older geologists called fan- 

 sliapcd stratification. Some such are only apparent, and are readily re- 

 solvable into separate folds. Others cannot be thus explained, and are 

 regarded as anticlinal, with the arch removed by denudation, 7. We 

 find several examples of anticlinals on a gigantic scale. Should these 

 be regarded as a single fold, thicknesses of 80,000 or 100,000 feet of 

 strata must necessarily exist. A careful study of some of them suggests 

 an origin from a double basin. The rising up of an older rock along 

 the centre induces the anticlinal structure, — the ge-anticlinal of Dana, — 

 and a continuance of the pressure produces on each side of the middle 

 a series of inversions, each set turned outwardly from the central line 

 (see p. 253); or the central line may be occupied by an eruptive rock. 

 This will induce, in connection with subsequent lateral force, the same 

 complicated anticlinal arrangement, while it will allow the belief in the 

 original basin shape of the formation, now appearing so very different. 

 We have, therefore, a basin that has the shape of a ridge for a part of its 

 area ; but in such a case there should be a closely compressed synclinal 

 upon each border. 8. A somewhat different structure will result when 

 there has been a sinking instead of a rising along the middle line, — the 

 undulation called geo-synclinal by Dana. 9. The double-basin structure 

 in its simplest form is one that has been very often invoked to explain 

 the position of our formations, particularly in Chapter IX. Its frequency 

 of occurrence, and the ease with which it can be made to explain the fan- 

 shaped and complex anticlinal structure, show it to be the normal condi- 

 tion of flexure in our state. Other important structural features might 

 be specified. 



I will endeavor next to ascertain the mutual relations of several of the 

 metamorphic groups, commencing with what seems to be very evident. 



