54 ON MESOZOIC DOLERITE AND DIABASE. 



preceding remarks it is hardly necessary to say that these- 

 rocks were never in the form of a lava over-spreading the 

 land in the presence of the atmosphere. They have been, 

 undeniably produced hj the crystallisation of a magma 

 which was injected or intruded into strata lying below the 

 surface. They have not crystallised rapidly, but under the 

 jjressure of suiDerincumbent rocks, which we seem com- 

 pelled to believe have been carried away by subsequent 

 denudation. There is absolutely nothing to show that 

 they ever succeeded in establishing communication with 

 the surface. If, however, they did, both the pipes by 

 which the magma ascended, and the basaltic flows in which 

 that ascent finally resulted, have been wasted, without 

 leaving a trace behind. The entire absence of mesozoic 

 basalts in the island suggests that these dolerites always 

 were subterranean, and that the faces and cliffs which we 

 now see are subterranean sections lifted for our inspection 

 by one or other of the earth movements, which geological 

 science so often reports. 



The names by which this eruptive rock is known are 

 not constant, and the discussion of them introduces us 

 to controversial petrology. The rock is that which is 

 called diabase in Euroj^e and Amei'ica, and dolerite by 

 most English petrographers. It is a plagioclase-pyroxene 

 rock, with the ophitic (and intersertal) structure which 

 so eminently characterises diabase that it has given rise 

 to the tei'm "diabasic structure." The rocks related to 

 it are, on one side, ophitic gabbros ; on the other, inter- 

 sertal basalts. In former dayp, if the rock was of pre- 

 tertiary age, it was called "diabase": if more j.'ecent,. 

 " dolerite." European geologists, however, reserved the 

 term " dolerite " for the coarse interior part of thick lava 

 sheets. This term is not much used now-a-days by 

 Continental petrographers, and the instances in which 

 it is applicable are considered by them as local and 

 unimportant. But the habit of attributing much impor- 

 tance to geological age as a factor in rock nomen- 

 clature has now died a natural death, even in Germany, 

 and the only question at issue is that of convenience- 

 Some general agreement is desirable as to whether the 

 present group should be called dolerite or diabase. English 

 petrographers (with the exception of Mr. Harker) use 

 the group name dolerite, and keep diabase for altered 

 varieties of the same rock. This usage was established. 

 by the late Mr. Allport, and has been followed 

 by Judd, Teall, Hatch, Rutley. Mr. Harker alone 



