EOCKS OF THE ESSEX DEIFT. 



353 



these, the former consists chiefly of plagioclase and hornblende, with 

 some good examples of magnesia-mica (biotite), and is therefore an 

 orthoclase-plagioclase syenite, and the latter is a pinkish-grey rock 

 with not much hornblende, and that in specks. It resembles a fine- 

 grained granite, and appears to be like the rock described by' Mr. Rutley 

 in his article " On the Igneous Hocks of the Warwickshire Coal-field' 

 (Geol. Mag. December 1888, p. 559), though there is not much plagio- 

 clase in the section which I have taken. 



Quartz-porphy rites. — These rocks seem to abound in the drift. 

 The specimens I have are numbered 3, 5, 6, 7, 110, 111, 112, 115 ; 

 the character of the last four being so difficult to determine that I had 

 at first placed them in another group ; but upon the whole they 

 appear to belong to the porphyritic quartz-rocks. Some of these 

 specimens are very close-grained and compact ; but in others the 

 porphyritic crystals are very distinct, and in two or three cases they 

 are so large and abundant as to give the rock the character of a 

 conglomerate. The ground-mass varies from a distinctly felsitic 

 character to a mosaic of small grains. The crystals of quartz are, 

 in some cases, mere rounded blebs ; while in others they are much 

 larger, with more or less distinct outline, though usually rounded. 

 They are frequently cracked, and, in almost all cases, are full of 

 minute enclosures and lines of dust. Crystals of felspar also occur 

 full of enclosures, much worn, and so decomposed as, in some cases, 

 to present a mealy appearance. In no. 7 there is an appearance of 

 fluxion-structure. 



Quartz-tourmaline. — Hocks of this class also seem to be very 

 abundant. The specimens are numbered 4, 8, 9, 10, 11, 12, 108, 

 109, 113, 114. The ground-mass is granitic ; but the grains vary 

 very much in size. They contain schorl in abundance, either in 

 needles enclosed in other crystals, or in aggregates of grains with a 

 decided tendency to a fan-shaped or radiate formation, or in spheroidal 

 patches, or else in long lines passing irregularly through the section 

 like thin threads. One of these sections (no. 113) seems to call for 

 special mention, as it is in many respects a remarkable rook. The 

 specimen is part of a large rounded boulder of very great, hardness, 

 which I dug out from the gravels ; it is perfectly smooth and highly 

 polished, and has very narrow almost parallel bands of yellow 

 alternating with narrow bands of black, so as to give it a peculiar 

 striped appearance. The fractured surface is quite dull and very 

 compact. The microscopic section shows a microcrystalline mosaic 

 of quartz, alternating with numerous dark bands composed of tour- 

 maline with dark amorphous matter. 



Felsites. — Of these rocks I have found only four specimens (nos. 14, 

 15, 16, 17), and these are all more or less devitrified. Nos. 14 and 

 15 are very dark compact rocks, of sp. gr. 2*65. The section of 

 no. 14 shows a spberulitic structure and fluxion-arrangement ; but 

 that of no. 15 is entirely spberulitic, for the spherulites press 

 so closely upon one another as to fill up the whole space. In 

 no. 16 there is a fairly wide band passing through the section, in which 

 crystallites are plainly discernible in long rods, and in many cases 



