/ 
S32 Br Brewster on the Optical Properties 
and Scotland. 7. The Dumbarton and Frisky Hall rocks, and 
some Renfrewshire ro<^ks. 8. Craig Lockhart. This remark- 
able rock has much the appearance of a volcanic tulfa, formed 
in some measure by means of volcanic water. 
*** Doubtful RocJcs. 
1. Castle Rock of Edinburgh and the Calton Hill. % Stir- 
ling Castle rock. 3. Salisbury Craig. 4. Blackford Hill. 
5. North Berwick Law, Traprain Law, and Girleton Hills. 
6. The rock of Rue Varey Point in Arran. 7. Some trap-rocks 
of Linlithgow. 8. The pitchstones of Arran. 9? and lastly, 
The amygdaloids and claystones. 
Art. XXII. — On the Optical Properties and Mechanical Con- 
ditkm of Amber. By David Brewster, LL, D. F.R.S. 
Lond. & Edin., &c. 
The nature and origin of Amber have afforded to natura- 
lists a fertile subject of controversy ; and, after all that has been 
written on the subject, it is difficult to say whether it is the pre- 
vailing opinion that amber is a mineral body, an indurated ve- 
getable j uice, or a mineral oil inspissated by chemical causes, or 
by the slow operation of time. 
The general resemblance of amber to Mellite^ which is a re- 
gularly crystallised body, and its occurrence at considerable 
depths in the earth, have induced most mineralogists to give it 
a place in their system next to this mineral : while its resem- 
blance to indurated gum§ ; its want of all appearance of crystal- 
line structure ; its occurrence in a stalactitical form, in strata 
of half decomposed trees ; and the circumstance of its contain- 
ing insects and vegetable remains, have given no inconsiderable 
degree of probability to the opposite opinion. 
In my early experiments on Depolarisation, I found that ^m- 
ber possessed the particular organisation which gives bands of 
complementary colours by polarised light % and that it some- 
times had such a regular structure, as to exhibit distinct neutral 
^ See PUL Trans. 1814, p. 214, and 1815, p. ST. 
