178 Bcn'eirs — Geological Surrey — Geology of India. 



invanably associated with the underlying complex. The massif thus 

 refeiTcd to extends as an ellipse 250 miles long and 60 miles wide in 

 a direction parallel to the adjacent coast of the Bay of Bengal. 

 Togetlier with its covering of metamorphosed sediments it forms 

 a denuded plateau, 3,000 to 4,000 feet in altitude, which is completely 

 carved into hills and valleys. Messrs. Walker & Collins, writing from 

 Canada, and working on rock-specimens obtained by Mr. Middlemiss 

 a few years ago, describe the petrology of this interesting district. 



They commence by stating that the suite of specimens appears to 

 represent a metamorphic rock series resulting from various stages of 

 mingling of ultra-basic igneous and of argillaceous sedimentary 

 materials. From a mineralogical point of view their observations ai'e 

 especially interesting, but we may confine our own remarks on the 

 present occasion to the composition and development of the mineral 

 tSaiiphirine as it occurs in these Visagapatam rocks. Sapphirine, as 

 a mineral species, seems to have been originally recognized by 

 iSteenstrup from Fiskernas in Greenland, and it has been found to 

 occur abundantly in the area now under description. The authors 

 tell us that the true birthplace of this mineral, as evinced by the 

 Indian rock-specimens, is the metamorphic zone of the contact 

 between the ' khondalites ' and spinel-bearing magmatic segregations. 

 Tliis Sapphirine-bearing zone is the result of the intermixture of these 

 highly argillaceous schists with segregated spinel rock : the product is 

 rich in alumina, magnesia, and the oxides of iron, but poor in silica ; 

 lime is practically absent. Steensti'up's examination of the Saj^phirine 

 rocks at Fiskernas led to the same conclusions. 



A comjjarison of the respective composition of spinel, Sapphirine, 

 and sillimanite indicates the intermediate position that Sapphirine 

 bears to the other two minerals in all its constituents. This is 

 thought to substantiate the view that it is really a product of their 

 interaction. Thus from the pure aluminate of iron and magnesia, 

 viz. spinel, which may be regarded as the utmost limit of a basic 

 species, we arrive at Sapphirine with its 15 per cent, of silica, largely 

 replacing the iron-oxides of spinel, and thence to sillimanite, which is 

 simply a sub-silicate of alumina with a small amount of iron-oxide. 

 It is noteworthy that the amount of alumina is nearly the same in all 

 three minerals, ranging from 65 to a little under 60 per cent. In 

 addition to their notice of Sapphirine the authors describe certain 

 rock-types, containing cordierite, which appear to be new to petro- 

 graphy, whilst a form of pyroxene has been detected having the 

 pleochroism of hj^persthene, but with inclined extinction ; for this 

 variety the name clino-hypersthene is suggested. 



It will be remembered that Mr. Holland in 1901 described the 

 Sivamalai series of Elteolite (nepheline) Syenites and Corundum 

 Syenites of Southern India. On the present occasion we have a note 

 by ]\Ir. Walker on the Nepheline Syenites from the hill-tracts of 

 the Visagapatam district, based on specimens also collected by 

 Mr. Middlemiss. The field relationships of these rocks aj^pear to be 

 those of igneous intrusions, occurring along with other gueissoid 

 igneous types, which form a part of the great boss extending from the 

 Godavari to the Mahanadi, and, as we have already seen, constitute 



