Notices of Memoirs — J. J. H. TeaJl — Igneous Magmas. 553 



The depth to which the zones of decomposition occur vary very 

 greatly, even in a short distance, and bear no relation to the surface 

 contour or to the water-level of the country. At the same 

 time, when the matter is carefully examined, the reason is 

 apparently clear. 



The rocks of the district, except in the immediate vicinity of 

 the lodes, have undergone very little change, since they often 

 outcrop, forming bold rough hills, which are strewn with solid 

 masses of rock. When such is the case, and a lode-cap is cut, 

 upon sinking to, say, 50 feet from the surface in the solid ground, 

 it is found to have undergone little or no altei'ation, which is clearly 

 due to the protection it has received by the 50 feet of solid rock 

 above it that may be said to have hermetically sealed it. 



On the other hand, when large rich lodes outcrop, the oxidizing 

 action has followed gradually down the same channels, up which 

 the mineral matter found its wa}', and has altered it to a con- 

 siderable depth ; whilst in those portions which were poor, and 

 not so highly mineralized, little or no change has taken place, even 

 in the same lode and at the same levels. 



The gold in the oxidized zone is clearly derived from the 

 decomposition of the tellnrides, since the gradual change from one 

 into the other has been traced, whilst, further, the pyrites proves to 

 contain little or no o-old. 



II. — Differentiation in Igneous Magmas as a Eesult of Pro- 

 gressive Crystallization.^ By J. J. H. Teall, M.A., F.R.S. 



CKTSTAL building in an originally homogeneous igneous magma 

 necessarily produces differentiation into portions of different 

 chemical composition, a fact the importance of which was first 

 impressed upon the author sixteen years ago in studying the 

 andesitic lavas and their associated quartz-porphyry dykes in the 

 Cheviot district. 



As is well known, Professor Rosenbusch has classified the common 

 constituents of igneous rocks into (1) the ores and accessory con- 

 stituents (including magnetite, etc.), (2) the ferro-magnesian con- 

 stituents, (3) the felspathic constituents, (4) free silica; and has 

 maintained that members of group (1) are the first to form in the 

 process of crystallization, and that while there are irregularities of 

 order between members of group (2) as compai-ed with those of 

 group (3), yet the members of these groups separate out inter se in 

 the order of increasing acidity. This order of crystallization has 

 been emphasized by man}'^ writers, though it has also been clearly 

 recognized that the law is not constant in different magmas and 

 under different conditions. The object of the present communication 

 is to call attention to what is at least an important exception to 

 this law. 



Among an extensive series of rocks and fossils collected by the 



' Abstract of a paper read at the Britisli Association, Section C (Geology), Toronto 

 Meeting, 1897. 



