438 Transactions. 



In hand-specimens the rock is very coarsely crystalline, 

 large crystals of feldspar, both orthoclase and plagioclase, being 

 visible. Microscopically the rock is holocrystalline and verv 

 coarse in structure. Plates of orthoclase, often with Carlsbad 

 twinning and polysynthetically twinned plagioclase, make up 

 the bulk of the rock. The plagioclase gives a mean extinction 

 angle on the albite twinning plane of 10J°, and is therefore a 

 variety of oligoclase. Dark patches of a granular nature con- 

 sisting of magnetite grains occur in elongated shape throughout, 

 often roughly assuming an amphibole form. These are pene- 

 trated by clear needles of apatite. These show another example 

 of resorption zones. The outline of many of the granular masses 

 is distinctly hornblendic, and the inclusion of apatite, a mineral 

 often found piercing crystals of hornblende, points to the original 

 existence of that mineral in the rock. Reaction of the magma 

 has corroded the crystals, leaving only the magnetite and apa- 

 tite, which evidently resisted the action. Differing from the 

 svenite, there are no remaining cores of hornblende in the rock. 



The naming of this rock is somewhat problematical. Origin- 

 ally .possessing amphibole, orthoclase, and oligoclase, together 

 with its non-porphyritic holocrystalline and coarse nature, its 

 characters would class it Math the Plauen'cher grund type of 

 Brogger. Among the hypabyssal rocks to which it belongs there 

 is no rock to which it corresponds. The entire absence of 

 trachytic structure, and its coarse nature and evenly developed 

 crystals, render it different from any of the types of porphvries. 

 Tn view of its almost wholly feldspathic nature, and the mixture 

 of feldspars which clearly places it in the porphyries, it seems 

 to be a coaTse example of a bostonite. 



The occurrence of the resorption zones in this as in the 

 syenite seems to point to its intrusion into a still-hot bostonite 

 lava. This surrounding hot material kept the magma sur- 

 rounding the first-formed crystals, the hornblende, in a liquid 

 condition, giving it time to act on the crystals, producing the 

 resorption of them. 



Though the name " bostonite " has been given to the rock, 

 it is only on account of its association of feldspars, almost entirely 

 feldspathic composition, and occurrence in a dyke, and not on 

 account of its resemblance to typical bostonites. 



Tinguaite. (Plate XI, fig. 6 ; Plate XII, fig. 7.) 

 A dyke of this occurs as indicated on the map. Its course 

 can be traced by small outcrops for a, distance of 3 or 4 chains, 

 though a determination of the width and dip cannot be made 

 In hand-specimens the rock is a dull-green, rather soft, and so 

 finely crystalline that no minerals can be distinguished. Under 



