in Reply to Mr. Brooke. 403 



of the other ingredients ; but there is not one of these which 

 is not, in more than one case, either absent, or present in very 

 small quantity. Shall we say then that garnet consists essen- 

 tially of silica, and that the other substances are accidental? 

 that it is thus chemically identical with quartz? This appears 

 too absurd to be thought of; and yet the doctrine of essential 

 composition with accidental mixture appears to leave us no 

 other alternative*. 



A case often quoted by the advocates of accidental mixture 

 is the Fontainbleau spar, and this Mr. Brooke adduces. This 

 instance has always appeared to me utterly irrelevant. The 

 carbonate of lime is here crystallized among the previously 

 existing particles of sand ; so that these are imbedded in it 

 like plums in a pudding. There is no difficulty in such a case, 

 any more than when any one mineral is crystallized about 

 splinters or needles of another ; — a very common occurrence. 

 But who would consider these splinters or needles as belong- 

 ing to the surrounding crystals ? In the Fontainbleau spar, 

 no one doubts that the carbonate of lime gives the crystalline 

 character, and that the quartz is caught up in it in grains. 

 The slightest touch of acid, or scratch with the penknife, 

 proves this. It is the easiest thing in the world to show that 

 this is not a homogeneous or simple mineral : the question is 

 about such as are homogeneous. 



Mr. Brooke would perhaps reply to this, that the Fontain- 

 bleau spar is a palpable mixture, and that other minerals may 

 be impalpable mixtures : that they may appear to be homo- 

 geneous when they are not really so : and that there may be 

 a gradual transition from mixtures obvious to the eye, like 

 the Fontainbleau spar, to mixtures which cannot be detected 

 by any physical character, and are discoverable only by che- 

 mical analysis. 



To this I reply, that if we really cannot tell whether a spe- 

 cimen be approximately a simple mineral, I do not see what 

 the use can be of analysing it at all. But the way to decide this 

 point, is to take minerals which possess the obvious qualities 

 which may be expected to characterize simple minerals; as 

 transparency, smooth surfaces, apparently uniform structure, 

 bright and uninterrupted cleavage extending to the smallest 

 fragments : all which properties the Fontainbleau spar wants. 

 And if we do take such cases, we neither find ourselves driven 

 to specimens manifestly very impure; nor do we find by che- 

 mical examination, large and irregular accidental mixtures. 

 Who ever detected, in apparently j)ure quartz, 10 or even 5 per 

 cent, of extraneous mixture? Who ever discovered in pellucid 



• Sec our present Number, p. 1^4. — Edit. 



;j F 2 calc- 



