THE HOUSATONIC VALLEY. 797 



Egremont Limestone, and at the south end a reversed fault with 

 the same rock over Cambrian Gneiss. It follows that the throw 

 varies most widely. At some fulcrum point, which must be near 

 the Maltby Quarry, this is practically nil. To The north of that 

 point, the western limb has been downthrown an amount which 

 steadily increases in going north, till in the vicinity of Sheffield 

 it can hardly be much less than a thousand feet. To the south- 

 ward of the Maltby Quarry, the western limb has been upthrown 

 and the amount of this upthrow at the Cobble must be several 

 hundred feet. 



The occurrence of two very thin quartzite lenses, which 

 follow a line parallel with the fault line along " Silver Street " in 

 Sheffield (Cf. Plate V.), is reason to believe that two secondary 

 faults there run parallel to the main fault. 



Additional evidence of the main overthrust is the occurrence 

 of numerous very large boulder-like masses of the tremolitic 

 quartzitic dolomite, resting on the Riga Schist to the east of the 

 road on the northeast flank of Miles Hill. It might be argued 

 that they are of glacial origin, since the direction of glacial move- 

 ment in this section is favorable, but they could only have come 

 from a point just across the river, and such masses are not dis- 

 tributed over the area to the southwest. Such masses are, how- 

 ever, found in abundance along the eastern side of the overthrust 

 for almost its entire length, and it therefore seems most probable 

 that they are fracture blocks produced in the faulting, which 

 have rounded through weathering, and as degradation has gone 

 on, have settled down upon lower beds of the mother rock, and 

 to some extent also upon the Riga Schist west of the river. 



This reversed fault presents some analogies with the over- 

 thrust faults of the southern Appalachians described by Hayes, 1 

 and those in New York described by Darton 2 , but the fault plane 



1 The Overthrust Faults of the Southern Appalachians, by C. W. Hayes. Bull. 

 Geol. Soc. Am., Vol. 2, pp. 141-154, pis. 2-3. Cf. also Willis and Hayes, Am. Jour. 

 Sci. (3) XLVI, pp. 257-268. Oct., 1893. 



2 On two Overthrusts in New York, by N. H. Darton. Bull. Geol. Soc. Am., Vol. 4, 

 pp. 436-439- 



