LOCAL CONDITIONS AFFECTING OBSERVATION. 101 



EFFECTS OF IGNEOUS INTRUSIONS. 



Ig'iieous intrusions, by inducing - lithological changes, often render the 

 determination of the equivalency of the altered strata a work of much 

 labor, particularly where the alteration is but one of several obstacles to 

 the tracing of the stratigraphy. Where, owing to unfavorable conditions 

 at the present surface, the contact of igneous rocks with stratified rocks can 

 not be observed, much perplexity often arises as to the order of events. 



METAMORPHISM. 



Dynamic metamorphism has changed both the texture and the colora- 

 tion of the rocks of extensive sections, thus masking the original sedimen- 

 tary characters and rendering the recognition of horizons, either by fossils 

 or by lithological peculiarities, difficult, Thus the Carboniferous strata 

 from Wickford north to Providence are highly metamorphosed, while beds 

 of the same age northeastward in the basin are very much less changed 

 from their original condition. Ottrelite-schists in the former area pass into 

 shales in the latter region. 



FOLDING AND FAULTING. 



While folds and faults directly aid investigation by bringing to the 

 surface strata which would otherwise be concealed, the complete inversion 

 of beds and the separation of blocks of strata by faulting result in con- 

 fusion. In the series of red strata which occur in North Attleboro the 

 utmost complication has arisen through the degree of folding; and here, 

 also, a block of strata of one series has been thrust into a position where 

 it is surrounded by beds of a different horizon. 



DENUDATION. 



Where beds are correlated from fold to fold by physical indications, 

 such as the repetition of like beds in the same order and with the same 

 topography, without the aid of fossils, denudation succeeded by local 

 concealment of parts of the series by deposition may lead to erroneous 

 conclusions in the matching of beds. In this basin there is a group of 

 conglomerates found only in synclinal areas; at lower horizons are other 

 conglomerate beds. If denudation should halt upon one of these lower 

 beds where it is exposed in the axis of a syncline, doubt would arise, 



