566 ' KEPOET— 1880. 



' glass-cavities/ there would often he no positive proof of tlieir Igneous origin. 

 There was also considerable doubt as to the manner in which certain minerals in 

 volcanic rocks had been generated. The observed facts were sufficient to prove 

 conclusively that some had been formed by sublimation, others by igneous fusion, 

 and others deposited from more or less highly heated water ; but it was difficult or 

 impossible to decide whether in particular cases certain minerals had been formed 

 exclusively by one or other process, or sometimes by one and sometimes by the 

 other, or by the combined action of water and a very high temperatiu"e. I must 

 confess that, even now that so much may be learned by studying with high magni- 

 fying powers the internal structure of crystals, I should hesitate very much in 

 deciding what were the exact conditions under which certain minerals have been 

 formed. This hesitation is probably as much due to inadequate examination and 

 to the want of a complete study of typical specimens, both in the field and by 

 means of the microscope, as to the unavoidable difficulties of the subject. Such 

 doubt, however, applies more to the origin of minerals occurring in cavities than 

 to those constituting a part of true rock-masses, to which latter I shall almost 

 exclusively refer on the present occasion. In the formation of these it appears to 

 me that sublimation has occurred to a very limited extent. In many cases true 

 igneous fusion has played such a leading part that the rocks may be fairly called 

 igneous, but in other cases, water, in some form or other, has, I think, had so much 

 influence that we should hesitate to call them ir/neous, and the term erupted would 

 be open to far less objection, since it would adequately express the manner of 

 their occurrence, and not commit us to anything open to serious doubt. 



In studying erupted rocks of different characters, we see that at one extreme they 

 are as truly igneous as any furnace-product, and, at the other extreme, hardly, if 

 at all, distinguishable from certain deposits met with in mineral veins, which 

 fiu-nish abundant evidence of the preponderating, if not exclusive, influence of 

 water, and have very little or nothing in common with products certainly known 

 to have been formed by the action of heat, and of heat alone. Between these 

 extremes there is every connecting link, and in certain cases it is almost, if not 

 quite impossible to say whether the characteristic structiu-e is due more to the 

 action of heat than of water. The great question is, whether the presence of a 

 small quantity of water in the liquid or gaseous state is the true cause of very well- 

 marked differences in structure ; or whether greater pressure, and the necessarily 

 slower rate of cooling, were not the more active causes, and the presence of water 

 in one state or another was merely the result of the same cause. This is a question 

 which ought to be solved by experiment ; but I fear it would be almost impossible 

 to perform the necessary operations in a satisfactcny manner. 



What I now propose to do is to describe a particular class of facts which have 

 lately attracted my attention, and to show that the crystalline minerals in products 

 known to have been formed bj' the action of heat alone, have a certain very well- 

 marked and characteristic structure, which is gradually modified as we pa^ss 

 through modern and more ancient volcanic to plutonic rocks, iu such a manner as to 

 show at once that they are intimately related, and yet differ in such characteristic 

 particulars that I think other agencies than mere heat must have had great 

 influence in producing the final result. 



In dealing with this subject, I propose, in the first place, to describe the 

 characteristic structure of products formed artificially under perfectly well-known 

 conditions, and then to pass gradually to that of rocks whose origin must be 

 inferred, and cannot be said to have been completely proved. 



Crystalline BloiD2npe Beads. 



Some years ago I devoted a considerable amount of time to the prepai'ation and 

 study of crystalline blowpipe beads, my aim being to discover simple and satis- 

 factory means for identifying small quantities of different earths and metalHc 

 oxides, when mixed with others ; and I never supposed that such small objects 

 would throw any light on the structure and origin of vast masses of natural rock. 

 The manner in which I prepared them was as follows : A small bead of borax was 



