206 SCIENCE PROGRESS. 



cylindrical form, the width usually diminishing downward. 

 The bounding walls exhibit a remarkable vertical striation. 

 The strata traversed have experienced no alteration at the 

 contact, except that they are upturned. The contents of 

 the pipe are fragmental materials, mainly an ultrabasic 

 breccia, which is the matrix of the diamonds. The phe- 

 nomena here summarised have been imitated experimentally 

 by Daubree by the use of high explosives, and his results 

 are, as he shows, of great geological importance. Their 

 application is not only to the drilling of cylindrical apertures, 

 but also to the comminution of rocks to produce volcanic 

 dust, the apparent plasticity of rock-masses under mechanical 

 forces, the upthrust of plugs of solid rock through vertical 

 perforations, and other geological phenomena. The author 

 named believes that some trachytic domes must have been 

 erupted in a nearly solid — not even pasty — condition. In 

 any case the connection between " necks " of breccia or 

 agglomerate and those consisting of plugs of solid rock is 

 a close one, the latter being doubtless due in many instances 

 to a neck formed of fragmental materials having subsequently 

 been invaded by an uprush of molten magma. The essential 

 feature of a true neck, as contrasted with the plug-like in- 

 trusions to be mentioned below, is that it represents what 

 has been an actual channel of communication between the 

 interior and the exterior of the earth's crust. 



Proceeding then to the forms of intrusive rocks, we con- 

 sider first, and more particularly, those characteristic of 

 regions where the disturbances of the earth's crust have 

 not been of a kind involving lateral thrust. The intruded 

 sheets, conveniently designated sills, are not confined to 

 areas exhibiting this type of structure, but they certainly 

 attain their highest development and show their greatest 

 regularity among flat strata, or more accurately, among 

 strata which have not experienced any marked folding re- 

 ferable to the period of the intrusions or earlier. One 

 obvious reason for this is that folding would tend to seal the 

 original partings of the strata in the limbs of the folds, while 

 concurrent fracture would open an easier channel for the 

 molten magma. The absence or rarity of dykes in many 



