WIDE AREA OF REGIONAL JOINTS 501 



be found insufficient for the satisfactory correlation throughout large 

 areas. There are, however, some indications that they will be found to 

 be more generally in accord than has usually been supposed. 



The most comprehensive studies of the joint directions which have 

 been made in the United States are those by Shaler and Tarr* in Massa- 

 chusetts, and by Buckley f in Wisconsin. The first mentioned study is 

 a comprehensive investigation and elaborate averaging of all joints which 

 were observed within a small district of complex structure; the last 

 mentioned, on the other hand, a comparison of the master joints found 

 at numerous widely separated localities with#n a large area in Wisconsin. 

 Shaler and Tarr in the Cape Ann district found that the joints are included 

 in a large number of series and are quite uniformly spaced within each 

 series. Measurements of the joint intervals were in many cases made. 

 For the area as a whole, it is found that the greater number of joints 

 trend in the directions north 20 degrees-25 degrees west, north ± 90 

 degrees east, north 30-35 degrees east, north 45-50 degrees east, north to 

 north 5 degrees west, north dz 15 degrees east, and north 30-35 degrees 

 west, the order named being the order of their relative abundance. Joint 

 directions were taken with a compass and measured to the nearest 5 

 degrees. It was found further that certain joint directions especially 

 characterized particular stretches of shore line, and the report adds : 



"The joints of this section, particularly those which strike N. 43 degrees E., are 

 remarkable for the continuity of the individual planes. In some cases they may 

 be observed extending on the shore line for the distance of 300 feet or more." . . . 



"The foregoing survey of the shore from the point of view of joint planes, 

 makes it tolerably evident that there is some relation between the salients and re- 

 entrants of the coast line and the jointed character of the rocks. A study of the 

 field would show the student that this is the case more effectively than it is indi- 

 cated to him by the merely cursory account which we have given him." 



Investigations of another great quarry region in Massachusetts by 

 Crosby J show that the planes of jointing fall into approximately verti- 

 cal and parallel series, which are ascribed by the author to crustal move- 

 ments. Speaking of joints of the Churchill quarry in Quincy, Crosby 



says : 



"Although they trend in all directions, the prevailing trends, as throughout the 

 quarry district, are approximately north-south or east- west, agreeing in this re- 

 spect with the dikes." 



*N. S. Shaler : The geology of cape Ann, Massachusetts. Ninth Ann. Report U. S. Geol. Survey, 

 1887-1888, 1889, pp. 529-611. 



fE. R. Buckley: On the building and ornamental stones of Wisconsin. Bull, iv, Wisconsin 

 Geol. and Nat. Hist. Survey, 1898, pp. 45G-460, pi. lxix. 



JW. O. Crosby: Geology of the Boston basin, vol. i, part iii, The Blue Hills complex. Occa- 

 sional papers, Bost. Soc. Nat. Hist., IV, p. 340, pi. xvi. 



