ORIGIN OF KAXU 1!.\ N KICTS. 363 



as along the Kenigha River in Matatielc, but this is the excep- 

 tion, and in the same area there are flat-headed dykes which come 

 up vertically and stop dead at a certain horizon, the cavity occu- 

 pied by the dyke being apparently punched from beneath, as 

 Barrois described, for the granite bosses of Brittany; yet 

 " punched " is not a happy expression, as there is no crumbling 

 or evidence of force having been used along the edges of the 

 dyke-cavity. In the plate* accompanying this paper there is 

 some apparent evidence of force, as the sedimentary beds are 

 cracked parallel with the buried side of the laccolite, but these 

 cracks are due to the efifect of the contact; the sedimentary beds 

 were heated and permeated by hot water at the time of the 

 intrusion, and after the whole had cooled down the sedinientary 

 beds contracted and left these cracks as a result. If the dolerite 

 on which the native is standing is followed to the right, the clean 

 way in which the sedimentary beds abut on the igneous rock 

 leave no doubt Init that the dolerite has melted out its cavity, and 

 the complete integrity of the stratum above the laccolite shows 

 clearly that the sedimentary rocks have not been riven asunder 

 to let in the molten magma. If the other side of the laccolite 

 were shown, it would be exactly the same, and there is no possible 

 question of the dolerite having been thrust in. Occurrences such 

 as these illustrated in the plate make one ask : Has anv new 

 material come in at all ; have not the sedimentary rocks sustained 

 a sort of spontaneous melting along certain lines and over par- 

 ticular areas? I cannot conceive that such sJiould have been the 

 case: rather the explanation lies in the fact that the molten 

 material dissolves out the cavity it is to occupy, assimilates such 

 substances as are suitable, and passes out such as are unsuit- 

 able. Where the latter go to is a difficult question to answer, but 

 it is possible that if an igneous rock can follow certain lines for 

 scores of miles, jjrovide sufficient material to not only keep itself 

 above the melting-point, but also to melt up and incorporate the 

 surrounding rocks, there can also be a downward stream which 

 carries away the waste jiroducts. 



If the dolerite, then, has melted out the cavities it now 

 occupies, there cannot be an increase in volume in the whole area 

 traversed by the dolerite. But we have seen reason to believe 

 that the dolerite does imjilv the inbringing of a certain amount of 

 new material, so that an amount of substances dissolved from the 

 sedimentary rocks equivalent to the amount supplied as new 

 material by the dolerite must have gone somewhere and have 

 there collected. The somewhere can only be below the network 

 of dykes, and, from comparison with the Transvaal laccolite, 



* Small laccolite, Mazeppa Bay, Kentani. The dark rock on which 

 the native is standing is dolerite; the sedimentary rocks are Kentani 

 (Beaufort) Beds with Oudenodon fossils. The sedimentary beds on the 

 rigrht abut on the sloping surface of the dolerite, and the stratum above 

 the dolerite is perfectly intact, and has not been arched up bv the dolerite. 

 (Photograph bv Dr. A. VV. Rogers.) 



