402 



STKIKE AND DIP. 



Another rock containing scarcely any carbonate of lime may be seen a few rods east of 

 the railroad in East Dorset, and northerly through the town for nearly four miles. It is 

 a black argillaceous rock decomposing into a soft black sand. It is hence composed of 

 quartz, argillaceous matter and limestone. We have sometimes conjectured whether this 

 bed might not be a means of identifying the same layers of limestone at different locali- 

 ties in the same way as fossils. It is situated very near the top of the limestone next to 

 the quartz. Similar beds of rock are the various dark colored and brecciated limerocks 

 adjoining elsewhere the slate or quartz, as upon Mt. Eolus, Wallingford, Rutland, etc. 

 The resemblance, however, is slight. 



Divisional Planes. 



. The divisional planes in this group are abundant, though not as numerous as in many rocks. Yet very 

 few observations have been made respecting them. This department has been greatly neglected in the field 

 work of the survey. 



Near the west line of Cornwall there is a large ledge of coarse brown limestone, having a dip from 

 45-70 W. In it are three sets of jointed planes : N. 45 E., dipping 65 E.; N. 60 W., perpendicular ; 

 and N. 45 W., dipping 75 W. At Adair's marble quarry, in South Wallingford, where the strata dip 

 57 W., joints dip 20 E., which might be mistaken for strata by inexperienced eyes, as they are generally 

 about three feet apart, and are of great assistance to the quarrymen. In Danby Comers the joints dip 

 40 N. W. At Hitchcock's marble quarry, near Clarendon Springs, the joints run N. and S., and dip from 

 60 to 65 E. (A. D. H.) As a general fact, it may be stated that jointed planes cross the strata 

 nearly at right angles to the dip and strike. Where several systems traverse the strata, rhombohedral or 

 rectangular blocks are formed. One of the best examples of these rocks is in North Dorset, near a curious 

 dike of lithomarge. The blocks are rectangular, and are much employed for building purposes, being 

 already prepared by nature for the mason's hands. 



Cleavage planes occasionally may be seen in the limestone,but not commonly. At Arlington, about two 

 miles west of the railroad station, where the limestone dips 5 E., the cleavage planes dip 25 E. In this case 

 the rock is cleft by numerous seams. And in one of the marble quarries near the village of Arlington, there 

 are indications of a structure resembling cleavage, without any disposition to split in the direction of the 

 streaks, more than in any other. The strata dip 4 N.E., while these markings dip 25 N.W. They 

 correspond best with planes of lamination in distinction from the strata. That would make it necessary to 

 suppose that marble was formed precisely as sandstones a deposition of foreign materials mechanically over 

 a slope. This is a unique case, and we are not prepared to indicate its precise nature. At James Grove's 

 house, in Weybridge, the strata dip 10 E., while the cleavage planes dip 40 E. 



STRIKE AND DIP OF THE STRATA: 



Locality. 



North Adams, Mass., Tunnel, 

 North Adams, 



North Adams, Natural Bridge, 

 Pownal, Massachusetts line, 

 Pownal, south part, 

 Pownal, west part, 

 Pownal, north part, 

 Pownal, east part, 

 Pownal, north part, 



SOUTHERN TERRAIN. 



Strike. 



N.5W., 

 N. 10 E., 



N.80E., 

 N. 10 W., 



Dip. 



0-45 W., 

 20-40 E., 

 10 E., 

 30 E., 

 20 E., 

 14 N., 

 20 E., 



About N. and S., 25 E., 



About S.W., 



Observer. 

 C. H. H. 

 C. H. H. 

 C. H. H. 

 C. B. A. 

 C. B. A. 

 C. B. A. 

 C. B. A. 

 C. H. H. 

 C. H. H. 



