198 C. SCHUCHERT THE NORTH AMERICAN GEOSYNCLIXES 



to have been originally larger than they are now. Xor are the geosyn- 

 clines "at the limit between continents and oceans"' — that is, they are not 

 on the continental shelves. The monogeosynclines and polygeosynclines 

 are situated hundreds of miles within the continents, inside of border- 

 lands that are not mere "simple swellings." 



We therefore see that mesogeosynclines are not situated upon a conti- 

 nent, but that they, like the oceans, lie between the continents. Also that 

 their depth is oceanic in kind and their deposits range from elastics to 

 the finest abyssal oozes. The entombed faunas are almost always normal 

 marine ones, and even though they are often of shallow-water kinds, yet 

 great thicknesses of almost unfossiliferous limestones and dolomites 

 occur, suggesting for them abyssal waters. The mesogeosynclines have 

 ver}^ long histories, and their structures show a far more complicated 

 succession of orogenies and geanticlinal formations than do even the 

 polygeosynclines ; they are ever so much greater in area, depth of water, 

 and in complicated structure than the monogeosynclines. On the other 

 hand, it does not appear from what is known of the mesogeosynclines 

 that they were ever completely drained of their waters, as was commonly 

 the case with the monogeosynclines and polygeosynclines. In other 

 words, the geosynclines within the continents reveal many more breaks 

 in sedimentation than do the mesogeosynclines that lie between the 

 continents. 



The depth of water in Tethys appears to have been very unlike in dif- 

 ferent places. The Eoman Mediterranean still has a length of about 

 2,200 miles and a greater width of over 1,000 miles. Its average depth 

 is about 4,500 feet, while the western deep goes to 12,200 and the eastern 

 one to 14,700 feet. Asiatic Tethys from Turkey to Burma is now moun- 

 tain land, and its Paleozoic deposits indicate deeper waters than those of 

 the Mesozoic, while the Cenozoic formations are essentially shallow-water 

 elastics; none of the deposits, however, suggest abyssal depths, as do 

 some of the formations of western Tethys. In late Paleozoic and Meso- 

 zoic times the eastern Tethyian mesogeosyncline extended across southern 

 Burma and Siam, through the Dutch East Indies, to Xew Guinea. This 

 eastern extension of greater Tethys, along with its folded and decidedly 

 overthrusted mountains, has been subsiding into the oceanic abyss since 

 early Cretaceous time ; here, too, the sediments with their oceanic faunas 

 often also suggest very deep waters. 



These facts and inferences indicate that tlie mesogeosynclines are 

 oceanic in character, duration, and geologic work accomplished, that they 

 clearly are not comparable to monogeosynclines, and only in a general 

 Avay are like polygeosynclines. 



