WILSON: MEDFOKD DIKE AKEA. 
365 
diabase ; at Pine hill the quarry openings indicate that the weather¬ 
ing has penetrated chiehy along joint cracks and to a depth much 
less than at Pasture hill. 'The difference in the amount of decay 
may be in part accounted for by the fact, that, where undisintegrated 
rock has been exposed at Pasture hill by the removal of the over- 
lying cover, it shows evidence of much jointing and slickensiding, 
phenomena which are almost absent in the great dike at Pine liill; 
such joints render the rock more permeable to atmospheric agents. 
A(je of the disintegration of the diabase. — The question at once 
arises, is the disintegrated diabase, as found to-day, residual material 
of preglacial or interglacial weathering, or has this great amount of 
disintegration taken place since the last glacial epoch ? To answer 
this question it will be necessary to consider in detail a numljer of 
facts as we find them to-day ; but first the opinions, as expressed in 
published papers, of three former workers in this field will be 
quoted. In a paper on “The Kaolin in Blandford, Mass.”, Pro¬ 
fessor W. O. Crosby states : — “ That the lithologic decay in this 
region [Blandford] during post-glacial time has been almost nil is 
proved by the fact that the removal of the drift exposes every¬ 
where, and on almost all kinds of rocks, a hard unaltered glaciated 
surface. I am well persuaded that even the decomposed dikes of 
diabase Avhich are observed in many parts of New England, and 
which are a feature of particular interest in the geologj^ of the 
Boston Basin, simply witness in general, to the substantial results 
accomplished by the quiet and unobtrusive atmospheric agents 
during the »long pre-glacial epochs, and owe their preservation to 
the narrow fissures which they occupy in harder and more durable 
rocks. This general conclusion appears to be justified by the fact 
that the great majority of the dikes present hard glaciated outcrops; 
and there is only now and then one which was so deeply decom¬ 
posed that the ice was unable wholly to remove the products of 
decay. It is undoubtedly true, however, that the more coarsely 
crystalline diabase forming the great dike in Medford, Somerville, 
etc., like some of the granites, is subject to rapid disintegration by 
frost action, and this process is now going on. We may thus 
readily explain the formation in post-glacial, and even in quite 
recent times, of the considerable bodies of coarse diabase sand 
known in this vicinity as Medford gravel and used for walks and 
carriage roads.” (’ 90 , p. 236.) Mr. Geo. P.’Merrill in discussing this 
