156 Stonehenge: the Petrology of its Stones. 
The fine obelisk No, 27 with the huge sarsen stone imminent over it, and 
ready to crush it,—as assuredly will happen at no very distant day if the sarsen 
megalith be not replaced in its erect position—does not differ petrologically 
from the other varieties of diabase at Stonehenge. ‘Ihe augite is perhaps a 
little less decomposed than usual, but this is not the case with the felspar which 
-ls very opaque and much metamorphosed. ‘The green pure chlorite seems a 
rare ingredient, while the gray flocculent chloritic mineral is present in the 
stone in the usual way. This stone is remarkable for its fine condition, the 
smooth roundness of its surface, and for a large hollow groove down its whole 
length either worked by art or ground out by some natural agency in its 
original rock-site. The hard tough character of this column of diabase has 
enabled it to resist the assaults of time, and even to a considerable degree the 
hammers of the iconoclasts; and it is now, as it probably was when Stonehenge 
was complete, the most remarkable of these smaller obeliskoid columns of 
foreign stone. Its pillar-like form, and the rounded smoothness as well of the 
convex surface as of the groove that runs down its side, may possibly have been 
the result of a quasi columnar structure in the original rock. Its position, 
on the other hand, at Stonehenge, may indicate that it had to perform the 
function of a solid base and support to some tall tree-stem, lashed into the 
groove, and used, it may be, to bear up an awning over the so-called altar~stone, 
or to carry aloft some accustomed signal. 
Of the four remaining stones of this class, Nos. 28, 29, 30, and 31, the for- 
mer two are similar to each other, and in the coarse grain of their felspar in- 
gredient resemble No. 2, while the remaining two are not remarkable for any 
special peculiarity. 
The four obeliskoid stones that remain to be described belong to 
quite another class of rock: a class to which it is much more difficult 
to assign a brief and descriptive petrological designation. To call 
it a hornstone, a felsite, or felstone, is only imperfectly to describe a | 
rock, which with the fluxion-structure of some igneous rocks com- 
bines indubitable evidence of its not having been a direct product of - 
igneous action. Its fundamental character is that of a very fine- 
grained mixture of quartz, felspar, and a grey chloritic mineral, in 
which fragments of rock are occasionally enclosed, while also, dis- 
tributed in the ground mass, there lie fragmentary skeleton crystals 
of felspar, which have almost entirely lost their original mineral 
composition ; their outlines being filled in with the chloritic mineral 
and often with the material of the ground mass itself. This quartzose 
ground mass exhibits the quartz in rounded microscopic grains, 
varying in their size, with which in variable proportions the felspathie 
particles and chloritic grains are mingled in such a way that the 
aspect of the rock is that of a triturated igneous rock, the materials 
