ASSOCIATED ROCKS OF THE LIZARD DISTRICT. 911 



probably some of the microliths in it are the pseudomorphs already 

 mentioned. As a type of the other variety, I take a specimen from 

 a dyke about four inches wide (just opposite the north end of a row 

 of white cottages), which cuts through two rather thin veins of 

 newer gabbro to a slight hummock of serpentine (I believe, no. 3 in 

 my description above). A hand specimen might very easily be 

 taken for a dull-coloured serpentine, were it not for its greater 

 hardness and its behaviour under the hammer. Microscopic exa- 

 mination shows it to be an altered basalt, infiltrated here and there 

 with serpentinous veins, full of minute crystals of plagioclase (or 

 probably a pseudomorph after it), occasionally about 0-02 inch long. 

 Here and there occur pale brownish films replacing the ordinary 

 dusky granulated matrix. Examination with polarized light shows 

 that actinolite is present here ; and minute bright specks over the 

 whole slide suggest the presence of the same mineral. The grains 

 of magnetite (perhaps also of ilmenite) are much decomposed. Here, 

 then, is a basalt, probably a magma basalt, greatly altered, but 

 still not in any true sense of the word converted into serpentine. 

 Another specimen, broken in 1873 from a very similar dyke, shows 

 a still greater change, the felspar crystals being still just discernible, 

 but the actinolite more characteristic and perfectly formed. 



Near the western end of the village, in the little open glen which 

 comes down to the cove, and about 100 yards from the sea, is 

 a quarry in serpentine. This (no. 13) is rather a dull dark 

 variet} r of an aluminous aspect, and is much cracked and jointed. 

 This is probably due to some small gabbro veins (coarsely crystalline 

 and much decomposed) which have been exposed in opening the 

 quarry. On the shore, east of a small row of houses, a mass of the 

 newer gabbro is exposed, very schistose in the upper part. Gabbro 

 veins may be seen on the hill-side behind the village. 



Proceeding onwards, we observe that the shore, where free from 

 sand, consists of serpentine with intrusive veins of the newer gabbro. 

 These become more numerous as we approach the main mass of 

 gabbro, which is reached about a quarter of a mile from the end of 

 the village. In plan this is an irregular oval, a little more than 

 four miles long and two wide, which forms the elevated upland of 

 of Crousa Down, some 300 feet above the sea. The line of the 

 raised beach is well marked by a low cliff, and in one place by a 

 projecting rocky plateau of considerable extent a short distance 

 above sea-level. The rock at the southern edge of the massif varies 

 greatly in texture, being sometimes moderately coarse (often, owing 

 to the prevailing greyish-blue tint, seeming from a short distance quite 

 compact), sometimes very coarse, with crystals of felspar and diallage 

 nearly an inch across. In the latter the felspar generally weathers 

 to a dull white ; the diallage is more or less metallic, commonly with 

 a greenish tinge. Some of the more compact varieties also weather 

 to a decided mottled white and dull green, like the gabbro of 

 Mont Colon and the Matterhorn (Pennine Alps); but others (for 

 what reason I cannot say) assume a slightly rusty tinge on their 

 surface as the felspar weathers out, leaving the diallage crystals 



