J. J. Harris Teall — Hornblende-Bearing Rocks. 351 



Messrs. Hague and Iddings l as occurring in the diorites and horn- 

 blende-andesites of the Washoe district. 



A specimen of a fine-gi'ained rock collected by Prof. Judd shows 

 a curious spotted or variolitic structure. The rock is much altered, 

 and it is therefore impossible to ascertain its original character. The 

 spots and the main mass of the rock both contain needle-shaped 

 pseudomorphs after hornblende and a microcrystalline ground-mass. 

 The hornblende-needles are not arranged in any definite manner in 

 relation to the spots ; sometimes a needle may be observed with one 

 end in a spot, and the other in the ground-mass of the rock. The 

 external boundaries of the spots are not defined by a sharp line, and 

 where two or more spots coalesce, there is no line of separation 

 between them. Where the spots are best defined, they consist of a 

 central more or less spherical portion, which is rendered nearly opaque 

 by opacite dust, and a marginal portion in which the ground-mass is 

 nearly colourless. The structure appears to be analogous to that of 

 the variolitic diabases. 



COMPAEISON OF THE EoCKS WITH ALLIED EoCKS OF OTHER 



Districts. 



The plagioclase-hornblende rocks form a very difficult and some- 

 what unsatisfactory group. In some, as, for example, the epidiorites 

 of Giimbel, the hornblende is undoubtedly secondary ; in others, in- 

 cluding the rocks now under consideration, it is certainly original. 

 As the question of the origin of the hornblende is one of the greatest 

 interest in relation to the past history of the rocks themselves 

 and of those portions of the earth's crust of which they form a 

 part, it becomes of importance to separate those in which horn- 

 blende is secondary from those in which it is a product of the 

 consolidation of an igneous magma. Mr. Allport 2 has shown 

 that in certain plagioclase-hornblende rocks surrounding the Land's 

 End mass of granite, the hornblende is wholly secondary after 

 augite. Such rocks have therefore no affinities with those now 

 under consideration. The same observer has, however, described 

 certain plagioclase-hornblende rocks (diorites) from Warwickshire, 3 

 in which the hornblende is mainly original. These rocks have 

 certain points of resemblance with those above referred to. The War- 

 wickshire rocks are, as a rule, much altered ; but Mr. Allport was 

 fortunate enough to obtain one specimen from Marston Jabet which 

 was absolutely fresh. It was mainly composed of long prisms of 

 well-formed hornblende set in a crystalline aggregate of plagioclase. 

 This rock, therefore, is closely allied to some of those described in 

 this paper. Another point of resemblance between the Warwick- 

 shire rocks and those from Assynt is the occurrence in both of a 

 colourless, or nearly colourless, pyroxene, in association with horn- 



1 On the Development of Crystallization in the Igneous Rocks of "Washoe, Nevada, 

 Bull. U. S. Geol Survey, No, 17, 1885. 



2 On the Metamorphic Rocks surrounding the Land's End Mass of Granite. 

 Q. J. G. S. vol. xxxii. p. 407. 



3 Q. J. G. S. vol. xxxv. p. 637. 



