54 
J. Y. ELSDEX—MICROSCOPIC STRUCTURE 
quartz is cloudy, and has many enclosed grains ; but neither distinct 
fluid cavities nor apatite needles were noticed. No plagioclase 
could he distinguished with certainty. 
Hitchin, 3 (PI. Y, f. 5).—A fine-grained, light-coloured gneiss 
with quartz and mica plainly visible. 
Beneath the microscope, orthoclastic felspar , with a few crystals 
of plagioclase, is seen. A brown cloudy alteration-product frequently 
obscures the crystals. Quartz, is present in irregular masses, often 
surrounded by mica. It contains cavities and a few enclosures, 
and a brownish decomposition-product has in some cases penetrated 
cracks in the granules. Mica is represented by biotite and muscovite , 
and some opaque matter, disseminated through the mass, may prob¬ 
ably be magnetite. No accessory minerals were noticed, but the 
whole rock appears brown owing to alteration. 
Ashwell, 150 (PI. Y, f. 6).—A dark green schist. Foliation 
well marked by bands of a dark green, very fibrous, hornblendic 
mineral. 
Under the microscope the fibrous mineral appears to have the 
structure of actinolite, although some crystals of hornblende are 
present. Green plates and scales of chlorite are abundantly asso¬ 
ciated with the actinolite, which occurs in radiating crystals 
throughout the mass, as well as in the planes of foliation. The 
ground-mass appears to consist almost entirely of quartz granules, 
with here and there a cloudy mass, possibly of decomposed felspar. 
But little, if any, magnetite is visible. 
Thus out of 27 specimens examined, 17 belong to the type of 
basic igneous rocks, 3 to the intermediate porphyrites, 4 are 
granites, and 3 are metamorphic (gneiss and schist). This large 
proportion of dolerites and basalts has an important bearing upon 
the origin of the boulders. For, if we consider the distribution 
of igneous rocks in Great Britain, we find that the Lower Silurian 
volcanic rocks of Wales and the English Lake District are chiefly 
acid in character; while in those of newer Palseozoic age, in the 
Midland counties and South Scotland, basic and intermediate rocks 
predominate. We have also to consider the very small area of 
exposed igneous rocks in the Midlands, in comparison with the 
large extent occupied by porphyrites, dolerites, and basalts in the 
South of Scotland. Thus there is, in the very classification of these 
boulders of Hertfordshire, an indication that their origin should be 
sought rather in a northerly than in a westerly direction. This 
view is strengthened by the great dissimilarity between the 
boulders, as a whole, when compared with the igneous rocks of 
the most considerable, as well as the nearest of the Midland 
districts, viz. Charnwood Forest.^ 
I will now attempt to examine briefly how far the mineralogical 
constitution of the boulders appears to throw light upon their 
origin; but my remarks must be taken not so much as advocating 
any distinct theory as to that origin, but rather as suggesting 
* A typical collection of Charnwood rocks has been kindly sent to me by Mr. 
W. J. Harrison, F.G.S. 
