224 ANNALS NEW YORK ACADEMY OF SCIENCES 



ORIGIN OF HORNBLENDE SCHIST AND GRANODIORITE GNEISS 



Dr. A. A. Julien^^ in a paper on the amphibole schists of Manliattan 

 Island has given an excellent description of these rocks and their mode of 

 occurrence. He has also taken up a detailed discussion in regard to their 

 origin, and his conclusions have been quite generally accepted as being 

 correct. He believes that these rocks represent metamorphosed igneous 

 rocks of rather basic composition which were injected through fissures 

 and spread out parallel to the bedding planes of the mica schist in the 

 form of intrusive sheets or sills at a period prior to the folding of the 

 latter. 



The chemical composition of the hornblende schist furnishes veiy 

 strong evidence in favor of its igneous origin. It is that of a rather basic 

 igneous rock. The three analyses of massive igneous rocks, one of a dia- 

 base, another of a camptonite and a third of a diorite, which are given for 

 comparison in a previous paragraph, correspond very closely to that of 

 the hornblende schist. The hornblende schist from Scourie, Scotland, has 

 a texture corresponding very closely to that of the New York hornblende 

 schist. The diabase from Scourie, Scotland, has a diabasic texture. In 

 mineral composition, it consists of feldspar, augite, ilmenite, apatite and 

 small amounts of such secondary^ products as hornblende, chloritic min- 

 erals, quartz and pyrite. The camptonite from Salem Neck, Massachu- 

 setts, approaches the ophitic texture. The minerals present are horn- 

 blende, less pyroxene, occasional olivine, a labradorite feldspar, a little 

 orthoclase and some magnetite and, rarely, apatite. The diorite from 

 Hump Mountain, North Carolina, contains plagioclase, orthoclase, horn- 

 blende and minor amounts of quartz, biotite. magnetite and garnet. It is 

 readily conceivable that rocks of such mineralogical composition upon 

 undergoing intense dynamic metamorphism could and would be con- 

 verted into hornblende schist. The augite in the case of the diabase and 

 camptonite would naturally be converted into hornblende. Olivine would 

 not be staple under« such conditions and, if present, would disappear, 

 entering into the composition of some other ferromagnesian mineral. 



Another strong point in favor of an igneous origin for the hornblende 

 schist is the shai-p contact always found occurring between it and the 

 mica schist, with the absence of any signs of gradation of one into the 

 other. It has already been pointed out that wherever undoubted sedi- 

 mentary contacts occur in the district no such sharp contacts are found, 

 as for example, the gradation of limestone into mica schist or one type of 

 mica schist into another. 



33 Bull. Geol. Soc. Am.. Vol. 14, pp. 421-404. Wm. 



