﻿Vol. 64.] BASIC INTRUSION OF BAETESTEEE. 507 



gran uli tic and in large idiomorphic cr3'stals, is abundant. The 

 rock at the spot numbered 55 (PI. LII, fig. 2) is similar but 

 rather finer-grained, showing some approach to the structure of a 

 basalt. The felspar and augite (which is almost entirely 'granulitic ') 

 are very fresh ; and the olivine, which is very abundant and evenly 

 distributed, is completely serpentinized. A few well-terminated 

 and slightly ophitic augite-crystals occur. 



As the northern margin of the intrusion is approached, a doleritic 

 type very rich iu olivine is again met with, as at (38). At (39) a 

 beautiful type occurs, in which fresh augite forms about half the 

 bulk of the rock : this is one of the coarsest types met with, the 

 plagioclase-phenocrysts, which have a maximum extinction-angle of 

 35°, reaching a length of 2 millimetres. At the base of the quarry 

 near the northern margin, where the boundary of the dyke dips in 

 below the altered Old Eed, is a green type of dolerite (34) rich 

 in ilmenite. In all these sections, the felspar is in laths showing 

 twinning on the Carlsbad and albite-types, with maximum extinction- 

 angles of about 35 to 40° : it is probably a basic labradorite. 



(B) The Basalts. 



By far the largest portion of the dyke is formed by a fine-grained 

 basaltic rock which shows but little variation in a hand-specimen, 

 being a black, compact, usually fresh basalt of uniform grain, with 

 small felspars and dark augites figuring prominently in it. 



A series of sections was examined, taken at intervals across the 

 whole thickness of the basalt, and showed the rock to be very 

 uniform in character except for the presence or absence of olivine. 



Four sections, two (1.1) from the actual junction with the 

 principal southern mass of dolerite, and the others (12 & 13) from 

 a distance of 2 and 3 inches respectively from the junction, all 

 agree as to the character of the ground-mass, which consists of little 

 plagioclase-needles associated with augite and magnetite-grains, 

 both minerals occurring in great abundance. All these sections 

 also agree in the character of the felspars, which form laths 

 having an average length of '5 to "75 millimetre, and giving 

 extinction -angles that suggest labradorite of a less basic type than 

 in the dolerite. The augite, however, varies considerably in the 

 diff'erent slides. In sections 11 (PI. LII, fig. 4) and 12 the augite 

 is almost entirely in the form of fair-sized crystals with irregular, 

 sometimes indistinct, and often corroded margins, and presenting a 

 peculiar speckled appearance owing to the separation of magnetite. 

 Some of the augite, however, occurs in fresh, idiomorphic, well- 

 cleaved crystals with no separation of magnetite. This type of 

 augite is the only one represented in section 13. The sections 

 (11 & 12) cut from the marginal 2 inches of the basalt are quite 

 devoid of olivine ; section 13, however, contains abundant olivine, 

 some of it serpentinized, some of it wholly or partly replaced by a 

 fibrous, noticeably-pleochroic, and brilliantly-polarizing mineral, 

 which appears to agree closely with a mineral observed under 



Q. J. G. S. No. 256. 2 m 



