IxX PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



essentially of pyroxene and anorthite. Whilst the magnesian silicates 

 crystallize so readily after simple fusion, these latter substances would 

 only melt to vitreous and amorphous masses, and in order to become 

 crystalline would have needed the presence of water. 



Analogies between Meteorites and Terrestrial Rocks. — Not only are 

 the elementary substances contained in the meteorites all of them 

 known in our own globe, but the most abundant in both cases are 

 iron, silicon, and oxygen. The resemblance of the meteoric stones to 

 our eruptive rocks is especially observable in the few examples of the 

 aluminous class (Juvinas), which are parallelled by the lava formed 

 of anorthite and pyroxene, as that from Thjorsa in Iceland, and in 

 the great division of the magnesian stones which so closely resemble 

 peridote and the compound rock Iherzolite*. 



The most marked differences are, that, whilst protoxide of iron, 

 whether combined with silica or with chromic acid, is a constituent 

 in both cases, the magnetic oxide so common in our mundane rocks 

 is, as a rule, absent from meteorites, whilst, on the other hand, the 

 latter contain native iron, which is correspondingly wanting on the 

 surface of the globe. Moreover the remarkable phosphide of iron 

 and nickel so largely present in the meteoric irons is a compound 

 not occurring in our rocks, where we may suppose it more or less re- 

 presented by phosphates. 



The importance of tlie magnesian rocks of the peridote class is 

 witnessed by the presence of that mineral in almost the whole series 

 of the meteorites, and by the numerous phenomena which tell of its 

 existence in the interior of the globe. The basalts and dolerites of 

 many and distant countries have brought to the surface fragments 

 of that mineral, often left angular and looking as if they had been 

 broken by force from a deeply seated and preexistent mass. Peri- 

 dote or olivine forms a large portion of certain other intrusive rocks, 

 as the Iherzolite and the considerable masses of the compound termed 

 by Hochstetter, in ISTew Zealand, dunitef ; and to this we have to 

 add the family tie that closely connects it with serpentine. 



No doubt it is still a disputed point whether the crystalline grains 

 of olivine so commonly observable in basalt and in certain lavas 

 have or have not been crystallized, in situ, from the elements present 

 in the general melted magma. Yon Buch was the first to point out 

 several remarkable instances where it would appear that this mineral 

 had been carried off unmelted by the lava. In the island of Lancerote, 

 the jagged ridges of lava near Tinguaton exhibit abundant masses of 

 crystalline olivine, which, near the origin of eruption, are often as 

 large as a man's head, but become smaller and less noticeable 

 where the lava-streams tail off towards SubacoJ. Similar project- 

 ing lumps of olivine had been observed at Puen Cahente in Palma, 

 and in the Yivarais; and more recently, our medallist of to-day, 

 Mr. Scrope, has drawn attention, in his work on Yolcanos, to the 



* See Damour, ' Bull. Soc. Geol. de France,' ser. 2. vol. xix. 

 t This rock, formed of olivine and chromite, appears to be similar to the 

 meteorite of Chassigny. 



X Yon Buch, ' Canarische Inseln,' pp. 303-6. 



