SURROUNDING THE LAND'S-END MASS OF GRANITE. 419 



entire conversion into hornblende. Frequently, however, the pro- 

 duct of alteration is a dull green substance, which is very slightly or 

 not at all dichroic. In the first sections examined, the rather large 

 irregular plates suggested the idea that the pyroxenic mineral might 

 be diallage ; but the absence of regular cleavage-lines and the occur- 

 rence of characteristic forms in some of the smaller crystals render 

 it far more likely that it is common augite. 



A comparison with the beautiful gabbro of Crousa Down (" dial- 

 lage rock") on the one hand, and with several Cornish dolerites on 

 the other, also tends to confirm this opinion ; I would, however, by 

 no means venture to assert that diallage is never present in the more 

 coarsely crystalline parts of the rocks. Although the felspar has 

 usually suffered an extreme amount of alteration, there is neverthe- 

 less unquestionable evidence of its presence in the form of rather 

 large crystals. 



As they occasionally penetrate the augite, their original form is 

 perfectly preserved, and in two specimens of the Tolcarn rock there 

 are several prisms so slightly altered that they still exhibit a beau- 

 tiful striation; there can be no doubt, therefore, as to the true 

 character of the rock. 



Besides the minute green belonites and pseudomorphs after augite, 

 there are numerous larger blades and flakes of dull green hornblende 

 disseminated through the base, and also filling cavities and veins. 



Like all hornblende it exhibits colours in polarized light, and is 

 dichroic ; but the crystals differ in character from those occurring in 

 diorites and other igneous rocks, nor are they the same as those 

 forming the true hornblende-schist of the Lizard district for ex- 

 ample ; they have often the character of actinolite, and are fre- 

 quently aggregated in radiating groups composed of flat blades of 

 a bluish-green colour, and not very translucent ; occasionally, how- 

 ever, there are crystals quite similar to those in ordinary hornblende- 

 schist. 



In one specimen of the Tolcarn rock they fill small veins and 

 spaces which were evidently cavities in the originally felspathic or, 

 possibly, glassy matrix. 



In many places the rock has been much fractured, and fine cracks 

 have broken across the various constituents, and have been subse- 

 quently filled by actinolite, with here and there a little pyrites. One 

 vein has been filled with clear quartz, in which actinolite has also 

 been formed. 



A careful examination of the mode of occurrence here described 

 shows conclusively that all the hornblende of these rocks is a pro- 

 duct of alteration ; and it appears highly probable that a continuation 

 or modification of the process would produce a typical hornblende- 

 schist. 



Certain portions of these metamorphosed dolerites are, in fact, 

 half-formed hornblende-schists, which only require the addition of 

 foliation to render the transformation complete. 



In the Tolcarn rock and several others there are many patches of 

 a brown colour, and only dimly translucent ; they have a distinct 



