Notes on Monchiquite Dykes. 3 



The cross-courses on the tield whicli liave displaced the anticlines and 

 the slides have resulted from niovemients distinctly minor to the main 

 ones, and Mr. Dunn has spoken of dyke material filling the cross 

 fractures, but I have not been able to verify this. 



In such a parallel system of dykes as this, which has originated from 

 the same magma, we might expect the dykes to communicate across the 

 anticlines at different levels in the crust of the earth. The only pos- 

 sible instance of this that came under my notice was in the Koch's 

 Pioneer Mine, Long Gully (Garden Gully line), where a lava comes 

 from the west at the 120!)-f(iot level, and continues through the upper 

 levels to the surface, l>ut is not found in the lower levels. 



Tlie metaniorphism of the intruded series by the dykes is extremely 

 slight. Apart from the peculiar appearance sometimes seen in the 

 quartz reefs, noticed above by Mr. Whitelaw, the only evidence I can 

 record is the presence of sillinianite crystals in quartz associated with a 

 lava obtained from the Ironbark Mine. 



The penetration of these thin parallel sheets, persistent for miles in 

 length, through a tliick series of folded sedimentary rocks, is a fact 

 which is impressive. It has astonished previous observers, and caused 

 Rickai'd (5) to propound a fanciful theory of dyke intrusion based on an 

 unproved statement that the mobility of the lava is due to superheated 

 steam and not to intense heat. Such was rightly attacked byArgall(6) 

 Mr. Dunn (2) is inclined to imagine the presence of explosive forces. There 

 is little to be gained at present by speculation. The mechanics of dyke 

 intrusion is a difficult subject and one little known. It can therefore be 

 well left to a later date. 



The question, too, has been raised as to whether these dykes were 

 channels of supply for lava flows at the surface. Rickard was quite 

 sure that they were, because he erroneously connected the monchiquites 

 with the newer basalt, but Mr. Dunn thought probably not. It is not 

 uncommon to find a dyke stream fading out in the upper levels, and an 

 instance was recently noticed in the Koch's Pioneer Mine, where the 

 top of the dyke was largely a mass of sulphur. The presence of sulphur 

 now implies the presence originally of svdphurous gases which were 

 vmable to escape through a vent. In this connection it may be remem- 

 bered that large black biotite crystals are to be found in the lava in 

 places like Jones's .shaft, New Chum line (2), and the Victoria Consols 

 Mine. Small biotites are sometimes seen in the sections of rocks from 

 the other mines. The formation of biotite as a mineral is now known 

 to require either the presence of a '" mineraliser" like water vapour, or 

 pressure. Mineralisers would have escaped and the pressure would 

 have been established with the existing surface at the time of intrusion. 

 This helps us to think that these dykes were independent and self-con- 

 tained intrusive bodies. Yet on the other hand the nearest petrogra- 



