ROCKS OF THE ESSEX DRIFT. 



357 



from the Rowley Rag ; and. that nos. 64, 65, 66 have some 

 points of resemblance to a rock from Swinnerton Park and to a 

 boulder fonnd near Leicester which resembles that rock : but they 

 differ in one most important point, viz. that nos. 64, 65, 66 contain 

 porphyritic augite, remarkably well developed, whereas I under- 

 stand that neither the Swinnerton-Park basalt nor the Leicester 

 boulder contain porphyritic augite at all. These dolerites, more- 

 over, are totally unlike any of the known dolerites of the north 

 of England ; they are, however, remarkably similar to some Scandi- 

 navian rocks, and this is especially so in the case of those very 

 specimens, no. 64, 65, 66, which differ from the Central England 

 rocks, and in that very point in which they so differ, viz. in their 

 containiug porphyritic augite ; for upon comparing them with some 

 specimens sent me from Sweden and labelled Pilahall (Scanie), 

 (xustafsborg (Scanie), and Anneklef (Scanie), the general similarity 

 between them is in itself striking ; and besides this the sections show 

 that they are almost identical in the following points : — the clearness 

 of the olivine, the porphyritic development and sharpness of outline 

 of the augite, the peculiar greenish appearance in the centre of 

 many of the crystals of augite, the powdered arrrangement of the 

 magnetite, and the specific gravity 2*9. 



The fourth group of dolerites, nos. 84 to 91, includes some rocks 

 of considerable interest. They are of a dark grey colour, as a rule 

 very close-grained, some being less so and lighter in colour : they 

 are plagioclase-augite rocks of a subophitic texture ; the plagioclase 

 is usually in microliths and the augite in granular aggregates, though 

 in 88 and 89 it is in distinct crystals with fine examples of twinning 

 bands ; these specimens are so remarkably like the rocks of the Whin 

 Sill as almost to establish an identity, for not onty is there a very 

 great general likeness in the hand-specimens, but the sections show 

 that they are identical in several points : (1) the sections of the drift- 

 rocks contain white colourless augite in long prisms, (2) they have some 

 fine examples of a distinctly pleochroic mineral of the enstatite group 

 (hypersthene ?) altering into bastite, (3) they have many small 

 grains of biotite scattered throughout the sections, and (4) the spe- 

 cific gravity of nos. 86 and 88 is 2-93. Upon referring to Mr. Teall's 

 article upon the "Whin Sill (in Quart. Journ. Geol. Soc. vol. xl. 

 No. 160) it will be seen that the first three points are special 

 peculiarities of this rock, and that in three out of the six specimens 

 mentioned the specific gravity is 2-94. I understand, however, 

 that in Central Scotland there are rocks allied to the Whin Sill, and 

 that the Hunneberg rocks in Scandinavia have been shown to be 

 very similar to these rocks ; the two specimens from the Hunneberg 

 which I have are of a totally different character, so much so that 

 there is not-even a general likeness between them and the specimen 

 which I have of the Whin Sill. 



The fifth group contains three specimens, nos. 92, 93, 94, each of 

 which seems to call for special mention. No. 92 is a piece of an 

 exceedingly hard boulder of dolerite, measuring about a foot each 

 way ; the boulder is completely rounded and polished, but scarcely 



