IOWA ACADEMY OF SCIENCES. 23 



two square miles it is, economically, one of the most important occurrences in 

 Maryland. Extensive quarries have been opened in it a short distance from the 

 station of Woodstock, on the main line of the Baltimore & Ohio railroad. Mowhere 

 among the Maryland granites is the phenomenon of jointing better shown 

 than in the "east" quarry of the Woodstock area. The horizontal divisional 

 planes are particularly prominent, and at first glance give the impression of 

 true stratification. These principal joints extend for considerable distances. 

 They are crossed by numerous inclined and vertical planes of natural cleavage, 

 which areusually much less prominent than the major lines just alluded to. 



As to the origin of the joints, it seems probable that they are due to two causes; 

 in part to the contraction of the granite during its original cooling; in part to a 

 subjection to severe tortion. The latter force is in all probability in action at the 

 present time; for, as already stated, it would seem that, according.to McGee,* crus- 

 tal movement is in active operation in the Piedmont region at the present time. 

 The optical disturbances of the granitic quartzes of the district may also be due in 

 some degree to the same causes. The very marked rifting aUo points to the 

 same conclusion. That systematic jointing may actually arise from strains of this 

 kind has been satisfactorily proven experimentally by Daubreef and seems to find 

 full confirmation in other more extensive trials, as well as in the field. On 

 approaching the middle one of the three quarries in the Woodstock district fine 

 illustrations of the weatherinar of granite are encountered. The quarry ledge has 

 the appearance of a great wall of cyclopean masonry — layer upon layer of huge 

 blocks rising one upon another with the regularity and precision of human effects. 

 The separate blocks are more or less oblong in shape and often measure 15 to 20 

 feet in length, and from 2 to 6 feet in height; they are all more or less rounded, 

 the spaces between the different boulders being filled with incoherent graniti 

 sand derived from the decomposed edges and sides of the blocks. It is quite evi- 

 dent that the granitic mass was onginally everywhere jointed and that atmos- 

 pheric decay took place much faster on the edges and corners than on the flat sides 

 of the great fragments, thus quickly rounding and forming them into boulders 

 like those found throughout the drift areas. The sandy matrix is usually from 5 

 to 10 inches in thickness. The interior of the boulders is perfectly fresh and 

 affords the best of rock for building purposes. As decomposition progresses the 

 amount of interstitial sand greatly increases and the blocks become proportionately 

 smaller. 



Another interesting phase of rock decay shown in the same opening is the phe- 

 nomena of spherical sundering; in which huge boulders have thick concentric 

 shells which come off one after another, not very unlike the ditterent coats of an 

 onion. 



It is very likely that it was during the later stages of rock decay just mentioned, 

 within the glaciated region, that most of the drift boulders in nearly their present 

 outlines were dislodged and transported by the glaciers. And it would appear 

 probable then that the erratics had been previously prepared to a great extent 

 before removal rather than rounded from angular fragments during transportation. 



A still more advanced stage of disintegration is shown at Sykesville, where the 

 weathering effects of the granite presents some striking contrasts with that of the 

 Woodstock area and is much more complete. Decomposition extends from the 

 surface downward for 30 feet or more in places. The decayed portion preserves all 



♦Seventh Ann. Rep. U. S. Geol. Sur., p. 619. 1888. 

 tEtudes de geol. Exper., p. 300. 



