

OF CENTRAL AND EASTERN CORNWALL. 483 



very materially from that of the crystalline dolerites. It is to be 

 remarked, however, that in the case of these lavas potash occurs 

 in the form of traces only. 



I. This is a greenish amygdaloidal lava from Pentire Point, in 

 which the cavities are usually of the size of hemp-seed, and are con- 

 siderably elongated in the direction of its line of flow. Sections 

 examined under the microscope are seen to be largely composed of 

 plagioclase, with occasionally traces of unaltered augite, together 

 with viridite and chlorite, as well as a slightly dichroic mineral, 

 which is perhaps hornblendic, resulting, like the chlorite and viridite, 

 from the alteration of augite ; they also contain slender crystals of 

 apatite, pseudomorphs after ilmenite, and a considerable amount of 

 the greyish dust-like mineral often present in altered crystalline 

 rocks. This latter substance, which by transmitted light has a dark 

 grey tint, when examined as an opaque object appears either of a 

 light grey hue or perfectly white. 



The cavities in this lava have generally been filled either with 

 crystalline calcite, with a mixture of calcite and viridite, with 

 quartz and chlorite, or with matted greenish microlites, which are 

 often distinctly dichroic. Viridite occasionally first lined a con- 

 siderable portion of the interior of a vacuity, after which the filling- 

 up was continued with crystalline calcite. A material of a dark 

 green colour, which sometimes covered the interior of the vesicle 

 previous to the deposit of viridite, is perhaps due to the decom- 

 position of a volcanic glass. 



A cavity in this rock, filled with crystalline quartz and vermicular 

 chlorite, is represented (magnified 30 diameters) in fig. 2 (Plate XX.). 

 This quartz encloses numerous very minute and somewhat rounded 

 cavities, which are generally empty ; and in no case was a movable 

 bubble observed in those contained in any of the sections of this 

 rock. In some few cases the cavities have become filled with a mix- 

 ture of quartz, viridite, and granular pyrites. 



II. This specimen was obtained from a quarry on the left of the 

 main road entering the village of Port Isaac from the south, and 

 differs but little from the foregoing, excepting that it is somewhat 

 coarser in texture and rather more schistose in character. 



On a farm called Trevarthan, three and a half miles south-east of 

 Port Isaac, a quarry is worked upon a bed of lava of considerable 

 thickness ; the rock, however, is so decomposed that the crystals of 

 felspar have frequently become partially obliterated. 



At Bokelly, in the parish of St. Kew, and in various other loca- 

 lities in this district, greenish foliated rocks, presenting, in hand- 

 specimens, the appearance of indurated ash-beds, have been opened 

 upon for road-making materials. Their colour is identical with 

 that of the before-described lavas; and the vesicles which they 

 contain, although small, are very numerous. Crystals of felspar, 

 which have occasionally the transparency of sanidine, are sometimes 

 enclosed in this rock, a specimen of which, from Bokelly, afforded 

 on being analyzed, the following results (sp. gr. =2*82): — 



