ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 913 



the replacing hornblende, there appears some little difference in the 

 various aggregates — some (and these show no signs of diallage) 

 being more " matted " in texture, if I may use the phrase, than 

 others, and showing here and there a little of a non-doubly re- 

 fracting mineral rather like serpentine. It can hardly be supposed 

 that one part of the same mass of gabbro would be rich in olivine 

 and another at no great distance entirely without it ; hence I con- 

 clude that sometimes the olivine in a gabbro, instead of changing 

 to serpentine, becomes converted into hornblende : the requisite 

 silicate of lime must be supplied by the neighbouring felspar — a 

 thing not impossible when we see how it is often penetrated by these 

 new acicular crystals. 



There are some dykes in this mass of gabbro. Soon after getting 

 on it we come, as we clamber along the shore, to two small dykes, 

 running rather irregularly, of a sharp -jointed, rather splintery, very 

 compact dull-greyish or greenish rock, of a rather serpentinous 

 aspect, the larger generally about 18 inches, and the smaller about 15 

 inches wide. I have had a slide cut from the former. It proves to be a 

 basalt, chiefly consisting of small crystals of plagioclase and augite, 

 both rather altered, and rather poor in magnetite. I suspect that 

 much of the plagioclase is now a pseudomorph ; we have the irre- 

 gular low-tinted granular aspect in part of the field. A good deal of 

 the augite has undergone change, and should, perhaps, rather be 

 called uralite ; but that the rock has been a basalt I have no 

 doubt. There is a little apatite. A short distance further is a 

 dyke from one to four yards wide, which forms a little headland. 

 This is in parts very like an ordinary basalt or anamesite, with a 

 glistening surface and weathering brown. Parts of it are rather 

 porphj'ritic, having very white felspar crystals up to about | inch 

 long. I have had slides cut from this ; and on examination it proves 

 to be a basalt, the plagioclase being fairly well preserved, the augite 

 sometimes very characteristic, and a good deal of olivine with a 

 granulated dusky aspect like (though on a smaller scale) that 

 described above ; there is also a fair quantity of magnetite. The 

 most porphyritic variety of this rock, macroscopically, so closely 

 resembles the most porphyritic variety of the " diorite " at Poltesco, 

 ttat the specimens might easily be confused. 



Beyond this I observed two other dykes : — one resembling a basalt, 

 not more than a foot wide, in a little headland ; the other some 

 quarter of a mile further on, also like a basalt, very compact, splin- 

 tery in fracture, much cracked and jointed. The above-described 

 dykes lie well in the first mile of the gabbro. After following the 

 shore for about that distance — a slow and rather laborious pro- 

 cess — I was obliged, by want of time, to take a path along the low 

 cliff above, as I was anxious to examine the greenstone of St. 

 Keverne. 



This I was not able to do as completely as I had hoped ; for 

 where from the map I had expected to come well into the mass 

 (here coloured over a space nearly half a mile across), I was still on 

 gabbro, pierced by frequent veins of greenstone ; and I had not time 



