32 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. t ** M0M [ ^S£ 



565) that the overthrusting of the Thibetan segments and the emergence of the basic rocks 

 took place simultaneously. He attaches considerable importance to this area in his general 

 discussion of the green-rocks. Traces of such "Tertiary" igneous intercalations extend back 

 to Kargil, on the Indus in Central Kashmir. 



The Himalayas, lying farther to the southeast, are but slightly known. Several traverses 

 near Sikhim record sills of peridotite, but it is not until we reach Burma that it becomes 

 abundant. 



In Upper Burma there occurs a mass of peridotite of unknown age which was originally 

 surrounded by gabbro. Into these sodic veins were injected as a last consolidation-product 

 of the ultrabasic magma, but instead of being merely albitic, as is usually the case (Benson 

 '13), the veins contained also nepheline and inclusions of serpentine and gabbro. Bleek ('08) 

 considers that great pressure, preceding and accompanying the intrusion of a mass of granite 

 into the complex, resulted in the change of the gabbros into amphibole-schists and of the sodic 

 veins into jadeite. 2 



The greatest developments of ultrabasic rocks are in the region of the Arakan Yoma and 

 occur in elongated masses intrusive into the Triassic beds and also into rocks that are Upper 

 Cretaceous or Eocene (Theobald '73). Vredenburg ('10) classes these with the cretaceous 

 igneous rocks. 



In the Andaman Islands, according to Oldham ('85) and Tipper ('11), there are pre- 

 Tertiary jaspers, quartzites, and porcellanous limestone, like the Lower Cretaceous limestones 

 of Beloochistan. Serpentines and a little diorite are intrusive into these, and pebbles derived 

 from the serpentine and jaspers occur in the overlying Eocene conglomerate. The data are 

 not sufficient to permit one to estimate the intensity of the folding of the pre-Tertiary beds. 



On the west coast of Sumatra, near Padang, serpentines and gabbros again appear, intru- 

 sive in the Carboniferous rocks, but pre-Eocene in age (Verbeek '05, p. 53). 



In the islands of Mentawei and Sipora off the west coast the serpentines are pre-Miocene 

 (Traverso '95). They extend across the Straits of Sunda into western Java (Van Es, Jr. '16), 

 and there they include masses of Cretaceous Orbitolina limestone, but do not invade the Eocene 

 rocks, which lie discordantly on them (Verbeek and Fennema '96). Gabbros and diabase, 

 sometimes pillowy or amygdaloidal, occur with tuffs in the Cretaceous rocks of Java, in a series 

 also containing Jurassic radiolarite (Neithammer '09), and they appear again in Flores, Wetter, 

 Luang, Sumba, Rotti, Timor, Kisar Letti, Moa, and Babar (Brower '19). Wanner ('13) has 

 shown the occurrence of some of the alpine tectonic structures with great sheet-folding in Timor, 

 Molengraaff ('13) has confirmed this, but the detailed account of his investigations are not yet 

 available to the writer. In the island of Letti, which lies off the northeastern end of Timor, 

 Molengraaff ('14) has shown that the basement consists of folded Permian mudstones, sand- 

 stones, and greywackes, with limestones and interbedded basic lavas and tuffs. These are 

 greatly disturbed and metamorphosed and pass northward into crystalline schists. On the 

 northern side of the island there are masses of serpentine thrust concordantly into and over the 

 schist, and upon these and the adjacent schists there rest a long series of exotic blocks of crystal- 

 line schist, various volcanic rocks, and fossiliferous Permian, Triassic (Halobia beds of radio- 

 larian chert), Jurassic, Cretaceous, and Lower Miocene rocks. The majority of these rocks 

 have no analogues in the island itself, though some of them are similar to rocks occurring in situ 

 in the adjacent islands of Timor and Moa. Molengraaff believes that these are remnants of a 

 recumbent sheet that was once thrust over the serpentines, and thus analogous to the "klip- 

 pen" of the Swiss Alps. He rejects the idea that they may have been thrust up with the in- 

 trusive basic rocks, as suggested by Krafft ('02) for the exotic blocks of the Himalayas. His 

 section across central Timor cited by Brouwer ('19) shows analogous features. In Moa, 



• Du Toit ('18) has described a region in Natal where normal quartz-bearing aplites, which traversed a series of schists and extended therefrom 

 into intercalated masses of serpentine, are changed to plumasite, or corundum-oligoclase-rock. This, he thinks, is due to the desilicifying action 

 of the ultrabasic rock on the invading acid magma. The serpentine has become silicified, with the formation of talc, etc. It may be queried whether 

 the development of nepheline-rocks and jadeite instead of the usual albitic dykes may not be due likewise to a process of desilicification by the 

 invaded ultra basic rock for somelocal reason not at present apparent. 



