ROCKS OF THE ESSEX DEIET. 



355 



pleochroic mineral of the enstatite group altering into bastite, the 

 pleochroism being from pale green to brown. No. 38 contains a 

 considerable amount of hornblende in irregular crystals, some of 

 which is enclosed in the felspar. 



Trachytes. — I have fonnd a fair number of specimens of these 

 rocks among the stones on the surface, but I am very doubtful 

 whether they really belong to the drift or were imported, probably 

 in very early times, for use as millstones ; for I have found not only 

 a curious small millstone made from this rock, but some other pieces 

 also grooved and marked in an evidently artificial manner. A section 

 from the millstone was kindly examined by Professor Bonney and 

 pronounced by him to be very similar to the well-known Nieder- 

 mendig rock. They are vesicular rocks of a dark grey colour 

 approaching to black, and of a tr achy tic texture ; the sections vary, 

 no. 39 showing crystals of plagioclase and hornblende in a base 

 composed of microliths of felspar and grains of hornblende. No. 40 

 has a vitreous base, enclosing abundant augite, but not much horn- 

 blende ; in 43 the augite is again abundant and there are some fine 

 instances of zonal structure : in all these rocks the crystals of augite 

 and hornblende are surrounded by a distinct narrow border much 

 lighter in colour and showing no pleochroism ; under crossed nicols 

 both crystal and border extinguish together, but in some cases appear 

 to leave a narrow rim of light between the crystal and the border. 



Dolerites. — Rocks of this class are exceedingly abundant in the 

 drift, but they are all of a more or less fine-grained character ; all 

 the coarser dolerites and all those of a true ophitic character are 

 remarkable for their absence. Moreover, I have not as yet found any 

 specimens of columnar dolerites, though I understand that these 

 rocks are of very common occurrence in the drift in the east of 

 England ; but of those which I have found, some are in very large 

 blocks, much polished and rounded, but only a few retain any striae : 

 one of these blocks, which was dug up out of a ditch some few 

 years ago and taken to the farm where it now lies, measures roughly 

 3 ft. x 3 ft. X 1 ft. 4 in., it is much rounded and polished and 

 in one small part of it the striae are very clear ; another large block 

 was lately dug from a depth of several feet in a clay-pit, in 

 which the yellowish-white clay is full of whitened flints very plainly 

 striated, like those numbered 173, one of which came from this clay ; 

 the block was found broken or, rather, cracked into several large 

 fragments. The specimens of these dolerites are numbered 44 to 

 94, and may be divided into five groups : — -Die first group comprises 

 nos. 44 to 52 ; these are of a fine-grained character and a dark 

 greenish-grey colour, the crystals being just perceptible to the naked 

 e} e ; under the microscope the texture is seen to be subophitic : they 

 are plagioclase-augite-olivine rocks, the plagioclase being usually 

 in microliths or else in lath-shaped crystals ; the augite in aggregates 

 or distinct crystals, in some cases rather highl} r coloured ; the olivine 

 varying very much, being sometimes so much altered that nothing 

 but the outline remains the same, in other cases .being very fresh 

 and clear except along the cracks, and in some instances being in 



