Discoveries ^Huygens respecting Double R^r action. 167 
1. The Neptunian Trap-rocles are the same as those met with 
in the Hebrides, and in our trap districts in the Mainland of 
Scotland. 
The following are enumerated: 1. Trap porphyry. Clink- 
stone porphyry. 3. Claystone porphyry. 4. Hornstone por- 
phyry. 5. Pitchstone porphyry. 6. Obsidian porphyry. 7. Pearl- 
stone porphyry. 8. Wacke. 9. Amygdaloid. 10. Greenstone. 
11. Basalt. 12. Trap-tuff. 
2. The Volcanic-N eptunian rocks, those namely which Menge 
maintains are formed from hot springs, are basalt, lava, tuff 
and trap porphyry. These rocks are said to be readily distin- 
guished, even in hand-specimens, from true trap-rocks, and true 
volcanic rocks, by their external characters. 
3. The true Volcanic rocks are lava, of which there are three 
kinds. Compact, Vesicular, and Spumous, also trass and tuff. 
Neither of these two latter tribes of rocks have hitherto been 
discovered in Scotland. 
4. The Alluvial rocks are formed of debris from the three 
preceding classes. 
The Neptunian trap-rocks extend through Iceland from 
S. E. to N. W., and are intersected by a great body of vol- 
canic rocks, which runs from S. W. to N. E., and therefore 
cuts across the trap-rocks in the form of an enormous vein. The 
rocks formed by hot springs, called Volcianic-Neptunian Rocks, 
are not confined to any particular tract ; on the contrary, they 
are very generally distributed throughout Iceland. And, lastly, 
it appears, that all the perennial springs in Iceland flow from 
Neptunian trap-rocks, never from true Volcanic rocks ; but that 
hot springs issue from volcanic rocks. R. J. 
Art. XXX. — Historical Account of Discoveries respecting 
the Double Refraction and Polarisation of Light. Continued 
from Vol. i. p. 296. 
Period II. Containing the Discoveries of Christopher 
Huygens. 
A FEW years after the publication of Bartholinus’s work on 
Iceland crystal, the attention of the celebrated Huygens was 
directed to the subject of double refraction. Fie was induced to 
