WEATHEEIJSTG- OF DIABASE 



199 



Massacliusetts, to Spot Pond in Stoneham, and beyond.^ The 

 average annual temperature for the region is 48.60° Pahr., with 

 recorded extremes of — 13° and + 102°. The ground remains 

 frozen and often covered with snow for at least four months of 

 the year. The annual precipitation is 44.96 inches. The ro<3k 

 at the point selected for study (Medford) is a coarsely granu- 

 lar admixture of lath-shaped feldspar, black mica, augite, and 

 brown basaltic hornblende, with the usual sprinkling of apatite, 

 magnetite, and ilmenite. Secondary uralite, chlorite, biotite, 

 leueoxene, kaolin, ealcite, pyrite, and quartz are common.^ 



The rock has undergone extensive disintegration, giving rise 

 to loose sand and gravel of a deep brown color, in which lie 

 rounded boulders of all sizes of the still undecomposed material. 

 These boulders, as is usually the case, show a more or less con- 

 centric structure, from without inward, until a solid core of 

 unaltered diabase is met with. (See PL 17, and Fig. 2, PL 22.) 



A mechanical separation of the disintegrated material yielded 

 results as below : 



mm. in diameter 42.300% 



mm. in diameter 20.355 



mm. in diameter 12.723 



mm. in diameter 9.567 



mm. in diameter 4.907 



mm. in diameter 4.181 



mm. in diameter 1.128 



.01-.005 mm. in diameter 0.370 



9. Clay '* .005-.0001 mm. in diameter 1.670 



10. Loss at 110° C 0.660 



11. Loss on ignition 1.730 



1. 



Coarse gravel above 2 



2. 



Fine gravel ' 



' 2-1 



3. 



Coarse sand ' 



' 1-5 



4. 



Medium sand ' 



' .5-.25 



5. 



Fine sand ' 



* .25-.1 



6. 



Very fine sand ' 



' .1-.05 



7. 



Silt ' 



' .05-.01 



8. 



Fine silt * 



' .Ol-.OO, 



99.691% 



Of the above, the first three sizes could be easily recognized 

 by the unaided eyes, as composed of particles of a compound 

 nature. In number 4 the separation had gone a trifle farther, 

 though even here inspection with a pocket lens revealed the 

 compound nature of many of the granules, somewhat obscured 

 by the prevailing discoloration from the oxides of iron. It 



iSee Disintegration and Decomposition of Diabase at Medford, Massa- 

 clmsetts, by G. P. Merrill, Bull. Geol. Soc. of America, Vol. YII, 1896, pp. 

 349-362. 



*0n the Petrograpliie Characters of a Dike of Diabase in the Boston 

 Basin, by W. H. Hobbs, Bull. Mus. Comp. Zoology, Vol. XVI, JSTo. 1, 1888. 



