SORBY : OPTICAL CHARACTERS OF MINERALS. 
15 
but remarkable discrepancies between mathematical theory 
and observation. My chief object now is simply to point 
out what valuable facts may be learned respecting the nature 
of any mineral by looking through it with a microscope at 
a circular hole or rectangular grating. This is a totally 
different thing to magnifying the mineral itself, or to looking 
through the mineral at any distant object without a micro- 
scope. The success of the method depends entirely upon the 
optical conditions characteristic of a compound microscope. 
I have lately greatly improved the apparatus hitherto 
employed, but the examples already given will, I trust, 
serve to prove that even with the less perfect appliances, it 
was possible to identify in a very satisfactory manner many 
of the minerals met with in thin microscopical sections of 
rocks, and thus to determine their constitution with far 
more certainty than heretofore. 
THE GEOLOGICAL HISTORY OF EAST YORKSHIRE. 
BY THE REV. J. F. BLAKE, M.A., F.G.S. 
The county of Yorkshire is broadly divided by the great 
plain of York into two very distinct districts ; that to the 
west comprising the romantic scenery of the Pennine chain 
and its dependencies, while that to the east forms the feebler 
but not unimportant elevations of the moorlands and wolds. 
This eastern portion of the county is altogether of a later 
date in geological history than the western, and contrasts with 
it strongly in this, that while the latter is almost entirely 
composed of an extraordinary development of one formation, 
the former is made up of the collected fragments of man}', 
which severally, as it were, enter an appearance in the 
district, but contribute very variously to its actual substance. 
In this feature, indeed, lies the chief interest of its history. 
It is no interminable mass of similar rocks, varied only by 
