158 



has also found a rock in situ which appears to be an altered 

 conglomerate. 



Beyond the northern boundary of Bolton the second or 

 middle band of mica slate disappears, but reappears five miles 

 farther north , near the common boundary of Harvard and Ayer ; 

 and the granite, which begins to widen here, is brought, by the 

 disappearance of the mica slate, in contact with the argillite 

 on the west, being bordered by argillite on both sides in this part 

 of its course. Unless the mica slate has actually thinned out to 

 nothing here, which is impossible, because the displaced 

 portion of the same bed only one and a half miles to the east 

 is 4,000 or 5,000 feet thick, we can only account for this 

 contact of the granite and argillite by assuming the existence 

 of another fault, with the downthrow on the west. This fault 

 probably commences in the northern part of Bolton, where the 

 middle band of mica slate begins to disappear ; but only after 

 it enters Harvard does the downthrow become sufficient to 

 carry all the mica slate below the present surface. The prob- 

 able relations of this fault to the strata which it cuts and to 

 the fault on the east side of the granite are shown in PI. 3, fig. 

 2, which represents a section across the valley parallel with 

 the first section, but five miles farther north, cutting the con- 

 glomerate near Harvard Centre. 



It is now clear that the mica slate of the middle belt, in Har- 

 vard, was carried up on the wedge of granite and gneiss, and has 

 been removed by denudation. The fault on the west side of 

 the granite is much shorter southward than that on the east, 

 and the downthrow attending it appears to have been less. 

 These two faults account perfectly for the present relative po- 

 sitions of the granite, mica slate, argillite, and conglomerate, in 

 this part of the valley; and in the great disturbance, of which 

 these fractures are an index, we find a probable cause for the 

 alteration of the conglomerate and the conversion of much of 

 the gneiss along this line into granite. There are abundant 

 indications that some of these rocks were in a yielding con- 

 dition at the time of their disturbance ; and Mr. Burbank is 



