8 PEOCEEDINGS OF THE NATIONAL MUSEUM. vol.06, 



vague tendency of the coarse diabase masses to be aligned in the 

 east-west direction, which might indicate that the east- west fractures 

 had their inception during the final consolidation of the diabase. 

 In most of the cases where the coarse rock bears a strictly intrusive 

 relation to the inclosing diabase of normal grain the streaks are 

 either extremely ragged or irregular, or fill fractures of very flat 

 attitude, which seems to indicate that few if &nj of the steeply dip- 

 ping breaks were in existence at the time of their intrusion. The 

 aplitic dikes, how^ever, tend unmistakably to follow the east-west fis- 

 sures and conclusively date this series of fractures with the final mag- 

 matic stages when the products of differentiation at an accessible dis- 

 tance were still fluid enough to be forced along cracks. The aplites are 

 few in number, but many of them have been affected by a type of 

 hydro thermal alteration, which is probably indicative of high tempera- 

 ture. This alteration is not confined to the vicinity of the relatively 

 few cracks which contain aplitic material, but is conspicuous adjacent 

 to many other cracks where there is no igneous material and is rather 

 universally present contiguous to the east-west fissures and their 

 subsidiary cracks, the hornblendic veneer, which is a characteristic 

 of these fissures, being in all probability a manifestation of this hydro- 

 thermal process, as discussed in detail below. 



So far as observed, no aplitic injection follows the north-south 

 joint system, nor is there much high temperature alteration with 

 development of hornblende, secondary titanite, or other minerals con- 

 sidered as indicative of high temperature hydrothermal processes, 

 contiguous to fractures of this system. The diabantite is considered 

 to be a relatively late and low temperature mineral, and the earliest 

 vein mineral seen in the shear zones of the north-south system is 

 datolite. 



For these reasons the east-west system of fractures is assumed to be 

 older than the north-south joint system. Opposed to this conclusion 

 is the appearance that the north-south joints are truncated by the 

 strong east-west fractures. This is not a serious contradiction, how- 

 ever, since the fissures may have accommodated movements at inter- 

 vals down to the present. That repeated movements took place is 

 shown by the aplites, intruded along fractures, being sliced by sub- 

 sequent movements, the later cracks being filled with secondary 

 minerals. 



In the above the postulation of two distinct systems of stresses has 

 been inferred, a necessity more apparent than real. It is entirely 

 conceivable that strong compressive stresses operating in a north- 

 westerly or westerly direction and finding relief in the east-west frac- 

 tures might induce the strong jointing in a direction perpendicular 

 to the direction of compression. 



