as illustrative of Geology. 67 



in greater quantity than in those of later creation. A similar 

 connection is proved, as was already mentioned above, from a 

 comparison of the plutonic and volcanic rocks. We can, there- 

 fore, form an idea of their successive origin, without being re- 

 duced to the necessity of assuming a great inequality in the dis- 

 tribution of the substances contained in the nucleus of the earth. 

 But the analogy may be still farther traced ; for, even in the 

 composition of the slags that arise from the process of the pro- 

 duction of iron, there is a striking resemblance to the nature of 

 many volcanic rocks ; a resemblance which consists not in the 

 considerable amount of iron ingredients alone, but chiefly in a 

 certain silicate which is peculiar to those volcanic rocks, and 

 whose substitute is a very ordinary attendant on basaltic rocks. 

 The slags that are formed in the early period of the refining 

 process, usually called rohschlacheii, consist chiefly of combina- 

 tions of silica with protoxide of iron, and some other bases of 

 smaller quantity, which have a more or less close approximation 

 to the relations of simple silicates ; and there not unfrequently 

 appear with a fixed relative proportion of ingredients, crystalli- 

 sed slags, which consist chiefly of a simple silicate of the protox- 

 ide of iron, and harmonise with the chrysolite or olivine both in 

 a stoechiometrical relation of composition, and also in regard to 

 the system of crystallisation. With respect to their ingredients, 

 a difi^erence between the crystalline slags and the olivine consists 

 in the circumstance, that, while in the former, the protoxide of 

 iron predominates, in the latter, magnesia abounds. But it is 

 well known that a reciprocal substitution is effected, and while 

 in crystallised slags we not unfrequently meet with magnesia 

 joined to the protoxide of iron, a considerable quantity of the 

 protoxide of iron is always found in olivine. The close affinity 

 that subsists between the olivine and crystallised slags has found 

 a beautiful confirmation in the discovery of the hyalosiderite, 

 which likewise is found in basaltic rocks, and which, in regard 

 to its chemical constitution, occupies the middle place between 

 these two bodies above named, being composed of nearly equal 

 parts of protoxide of iron and magnesia. 



After the attempt to employ metaliurgic experiments in the 

 elucidation of the igneous process that lies profoundly concealed in 

 the bowels of the earth, it will appear less venturous, to found 



E 2 



