VI. 
ON THE MICROSCOPIC STRUCTURE OF BOULDERS FOUND IN 
THE NORTH OF HERTFORDSHIRE. 
By J. Vincent Elsden, B.Sc. (Bond.), F.C.S. 
Head at Watford , 15 th January, 1884. 
PLATES III—V. 
The microscopic structure of erratic blocks has a two-fold 
interest. In the first place it has a special importance as being 
perhaps the only means by which igneous rocks can be identified 
with any approach to certainty. Fragments of sedimentary rocks 
generally have either some special character, or some included 
fossil, by which their original source may be easily known; but 
boulders of granite, basalt, schist, or other crystalline material are 
often so similar in general appearance that it is impossible, in 
many cases, from a mere macroscopic comparison, to determine the 
locality whence they have been conveyed. When, however, their 
minute structure is examined, there are often found certain 
mineralogical peculiarities, which, small and insignificant as they 
may appear, are yet sufficiently constant and characteristic to 
enable their parent rocks to be recognised. 
In the second place, there is, in the study of rock-sections, a 
general interest which may perhaps be new to many members of 
a Society whose geological work lies so exclusively amongst the 
newer sedimentary deposits, and who may have, therefore, few 
opportunities of examining for themselves the various modes in 
which minerals aggregate into rock-masses, and the alterations 
which they experience by subsequent chemical action. 
I will now proceed to describe the chief features of some 
specimens of boulders from the north of Hertfordshire kindly 
placed at my disposal by Mr. H. Gf. Fordham, and will after¬ 
wards draw any conclusions that may seem legitimate respect¬ 
ing their origin and petrographical peculiarities. It must, how¬ 
ever, be remembered that a detailed examination of small weathered 
specimens is often a matter of considerable difficulty, and does 
not, in all cases, lead to such satisfactory conclusions as would 
be obtained from fresher samples. 
The names of the localities and the numbers quoted are from the 
labels on the pieces of boulders as sent to me by Mr. Fordham, and 
(except those in italics) are the same as in his paper (see p. 33 
ante) and in the Reports of the Boulder Committee of the British 
Association. 
1. Doleeites.^ 
Ashwell, 186 (PI. Ill, f. 1).—This is a moderately coarse, dark 
crystalline rock, with but little outward sign of decomposition : it 
contains visible crystals of felspar, pyroxene, and bright green 
grains of olivine. 
* I have tried to separate the dolerites from the basalts, notwithstanding the 
difficulty, in some cases, of distinguishing any true glassy matrix. 
VOL. III.—PART II. 
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